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U n i t 2 Optics and Vision
ight comes. You are ready to fall asleep. You turn off your bedside light, and the room is suddenly thrown into N darkness. For a moment, you cannot see a thing. What happens to the objects in your room? Why can’t you see them? Do they disappear? Do their colours turn to black? Do you need light to see them? Do objects have a light of their own? Optics is the scientific study of the properties of light and the way technology uses these properties. It is a A photographers’ gallery subject that has fascinated and puzzled people for centuries. In this unit, you will study the properties of visible light as well as those of other types of radiation. By answering Sunglasses affect vision. the questions above, you will broaden your understanding about the way light is produced, transmitted, and detected. You will learn that colours have special meaning for First Nations and Métis peoples and relate to the stages of life as shown in the colours of the medicine wheel.
BIG IDEAS
1.0 Light travels in straight lines and illuminance decreases with distance from its source. The rainbow is a natural splitting of light. 2.0 The law of reflection describes how light reflects from a plane mirror.
3.0 Light is refracted by transparent materials, and this is what makes lenses so useful.
4.0 The properties of light explain how the eye and the camera capture images.
5.0 The visible light spectrum is made up of different colours. Colours hold special meaning for First Nations and Métis peoples.
6.0 Visible light is only one part of the electromagnetic spectrum.
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InvitationInvitation toto ExploreExplore SKILLS CANADA SASKATCHEWAN
Wollaston Lake
Cree Lake Reindeer Lake
Montreal Lake Prince Albert North Sa R. sk n atchewa
Saskatoon
outh Sa . S sk R atchewan Regina Skills Canada Saskatchewan Moose Jaw provides students with chances to explore skilled occupations and to meet students from 100 Km their province, Canada, and around the world. Photos courtesy of WorldSkills International
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How can projects in secondary, post-secondary, and apprenticeship courses lead to light and graphics demonstrations in an
international competition? Skills Canada Saskatchewan could Photo courtesy WorldSkills International have the answer. Four Saskatchewan cities take turns to host the annual Olympic-style skills competitions that provide a forum for secondary, post-secondary, and apprentice students to compete to be the best in the province. Students compete in more than 25 trade and technology areas. Those who win at the Skills Canada Saskatchewan competition have an opportunity to compete at the Canadian Skills Competition. Every two years, In September 2009, 51 countries/ the top Canadian Skills competitors have an additional opportunity regions competed at the International WorldSkills Competition at Stampede to compete at a WorldSkills competition and to meet students Park in Calgary, Alberta. from many other countries. Skills Canada Saskatchewan’s mission is “to promote skilled trades and technologies as a first-choice career option for youth in Saskatchewan.” Skills Canada Saskatchewan directly involves industry in evaluating student performance at the competition. Photo courtesy WorldSkills International
Light, Optics, and Technology Light, colour, optics, and waves are used in the various trades and technologies that are associated with Skills Canada Saskatchewan. Electricians connect fluorescent and incandescent lights. Web designers use colourful graphics and animations in website layouts. Graphic artists use colour to design graphics for Welders wear protective eyewear print resources. Hairstylists use colour in a different way in hair to shield their eyes from radiation dye to colour or de-colour hair. Electronics technologists use produced from the welding arc. light-emitting diodes (LEDs), strobe lights, and black lights. Robotics specialists manoeuvre their robots using radio waves from a radio controller. With a partner or in a small group, brainstorm other uses of optics and light in careers in trades and technologies such as: • Computer Animation (2-D and 3-D) • Office Manager (using software applications) • IT PC/Network Support • Plumbing • Aesthetics Architectural technologists use • Baking • Autobody Repair computer monitors that combine colours to produce the image of • Industrial Control • Car Painting blueprints and to show 2-D and • Offset Printing • Painting 3-D structures.
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I NVESTIGATOR
Light Up Your Life
Before You Start Station C Working in groups, you will experiment • Put three coloured filters (blue, red, with light. At each station, answer the and green) separately over three light following questions. sources of equal brightness (three flashlights or ray boxes). Shine each 1 What do you observe? Explain coloured light source at a white screen. what you see. • Overlap (mix) two different coloured 2 What do you want to find out? lights together in different combinations. What happens? Mix all three coloured Station A lights. What happens? • Look at yourself in a concave mirror (the reflective surface is on the inside of the curve) and a convex mirror (the reflective surface bulges out). • What happens to your image in the mirror in each case?
Station D • Look at a pencil standing in a glass of water. Observe the pencil through the glass at different angles. What do Station B you notice? • See what happens to a beam of white light when it hits (or goes through) a prism. • What is a rainbow?
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Station E Station F • Look at a sheet of graph or lined paper • Shine a light source through a glass using first a lens that is thicker in the (transparent), tissue paper (translucent), middle than at the edges and then a and a book (opaque). lens that is thinner in the middle. • What happens to the light in each case? • What happens to the distances between the lines when you move • State what you think the terms each lens farther away from the paper? transparent, translucent, and opaque mean. • What happens when you move each lens closer to the paper?
Focus Your Thoughts
1 Look at the four photos to the right and create a title for each. Explain, to the best of your knowledge, how light or the properties of light account for each photo.
2 Below is a list of words whose definitions you may or may not know. Working with a partner, sort the words you know into groups. Predict how each group represents or demonstrates one of the key properties of light that you have explored so far. brightness rainbow energy reflection luminous colour refraction transparent concave mirror or lens translucent wavelength convex mirror or lens
3 Have you thought about a future career? Is it in the field of a skilled trade or technology? For this career, do you need to continue your education after grade 12? How might optics and light be involved in your career of choice?
4 Describe in a paragraph what you think are the properties of light energy. How did you know? How could you find out about any properties of light that you are not sure about?
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BBIGIG IDEAI DEAS Light travels in straight lines and illuminance 1.0 decreases with distance from its source.
The people using shadow puppets have something in common with the designers of the lighting for this new theatre. They both need to understand where light comes from and how light travels in order to create the effects they wish. How does light travel from its source? What evidence do you have that this is so?
Light travelling in straight lines can produce varied and beautiful effects.
1.1 How Does Light Travel?
What source of light are you using to read this page? Put your hand between this source and the page. What happens? Experiment to see how the shadow changes if you change the angle of your hand or its distance from the page. What do you conclude about how light travels from this source to your page?
Straight from Here to There You probably concluded (or knew from earlier studies) that light travels in straight lines from its source. Light cannot bend around objects. When you placed your hand between the light source and this page, it prevented the light from reaching the page and made a shadow. Your hand is an opaque material. Where is the Sun in this scene? Opaque materials, such as bricks, books, and people, reflect How do you know? (throw back) or absorb (take in) all the light that reaches them.
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A shadow is created whenever light hits an opaque material. A shadow is composed of two regions, a partially shaded region and a fully shaded region. The fully shaded region is known as the umbra (from the Latin word meaning shade). The partially umbra shaded region is known as the penumbra (from the Latin word meaning partial shade). The penumbra forms due to some of the light being blocked and some passing by the opaque object. penumbra See the table below for a review of some of the characteristics of objects and materials in relation to light.
A Light Vocabulary
Term Definition Examples Shadows have two regions. luminous objects that produce light Sun, light bulb, fire non-luminous objects that do not produce Moon, most objects on Earth light but may reflect it transparent materials that allow light to glass, air, your family’s old pass through photographic slide film opaque opaque materials that do not allow wood, metal, thick plastic light to pass through
translucent materials that allow some some types of cloth, stained glass transparent light to pass through
translucent You can see how light travels in a straight line when you use a flashlight or see a beam of sunlight coming down through a cloudy sky. You might wonder why, when you turn on the light A ray diagram illustrates the direction in a room, the light seems to reach almost everywhere. How is of the path of light in different media. that possible, if light travels in straight lines? One reason is that the light from the light source in your room travels outward in all directions. Another reason, which will be discussed later, is that light reflects in straight lines from the ceiling and walls. The lighting in a room is usually designed to allow light to reach all directions. A flashlight is designed to allow light to go in one direction. In either case, light travels in straight lines from its source. A directed straight line that represents the path followed by the light is called a ray. A ray diagram illustrates the direction of the path of light. Straight lines and an arrowhead indicate the direction the light travels.
Projecting Images Different structures direct the light in different ways. As you can see from the images on this page, light travels in straight lines from its source as well as in all directions. It also travels in straight lines after being reflected from an object. These characteristics of light should help you understand the next activity.
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P ROBLEM S OLVER
Make a Pinhole Camera
The pinhole camera uses a tiny hole to form an image. In one form or another, it has been used for centuries. Photography was invented using a large version of the pinhole camera called the camera obscura, which means “dark chamber.” This was a small dark room with a tiny pinhole at one end. In 1826, Nicéphore Niépce (pronounced “njeeps”) replaced the pinhole with a lens and projected an image onto a metal plate Niépce’s first photograph coated with light-sensitive chemicals. This image was probably the first photograph • Use black electrical tape, masking ever permanently recorded. It took about tape, or duct tape to join the hollow eight hours to expose the image! tube to the container at the end that The ancient Chinese and Greeks used has the lid. You now have a long the pinhole camera to look at solar cylinder with a pinhole at one end eclipses without endangering their eyes. and a screen (the lid) in the middle. By the time of Niépce, artists used this This is your pinhole camera. early form of camera to trace images onto • Point the pinhole of the camera at a canvas before beginning a painting. bright lamp or a light bulb.
The Question 1 Look in the open end. What do you see? How does a pinhole camera record an image? 2 What does this show you about how • Make your own pinhole camera using light travels? two empty chip containers (or any 3 Draw a ray diagram of the pinhole other cylindrical containers that camera. have metal bottoms). 4 Share and compare your observations • Remove the lid and bottom from the with your classmates. first container, leaving a hollow tube. • Use a nail to punch a very small hole in the bottom of the second container. Leave the lid on to act as a screen for the image.
CAUTION! Wear eye protection when using a hammer and nail. Do not use the pinhole camera to look directly at the Sun.
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1 a) What causes shadows to occur? b) How could you make two shadows from the same object at the same time? four shadows? c) Is there a limit to the number of different shadows you can make at once? Explain.
2 What do you notice about the shape of the beam of light from a flashlight or movie projector as the beam moves farther from its source? Why is this happening?
3 Compare the rabbit’s eyes with those of its predator, the hawk. Based on how light travels, which of these animals can see a person approaching from the side or behind? Why do you think this ability is important to this animal’s survival?
4 Explain why you agree or disagree with this statement: “Once light leaves its source, it will persist and keep going forever.” If you disagreed, under what conditions might this statement be true?
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1.2 Illuminance and Distance
Imagine there has been a power failure and all the lights in your home go out. You have a test tomorrow and need to study. You find a candle to use as a light source and place it so that you have enough light to be able to read your notes. Your sister shows up and wants to share the light so she can work on a puzzle. You move the candle to the middle of the table. When you look at your notes again, you find the pages are now too dim to be easily read. What happened to the light? How could you provide enough light for both of you?
Illuminance The amount of light arriving at one place per unit area is referred to as illuminance. Luminance intensity, also known as brightness, refers to the light that is emitted from the surface of an object such as a light or candle. A person sitting far away from a light source will notice that the light is dimmer, or the illuminance is less intense, than when she sits closer. In the following activity, you will explore what affects illuminance. Where would you find the greatest illuminance if you wanted to read a map by these streetlights at night?
P ROBLEM S OLVER
Light Power
You have probably used a flashlight in • Plan and perform an experiment to a darkened room or outside at night. show how illuminance is related to What have you noticed about the light that the distance light travels before shines? For instance, suppose you shone striking an object. a flashlight on the far wall in a darkened • Predict what you think will happen. gymnasium and then in a darkened small room. Would the illuminance (that is, the • If possible, compare several types of brightness of each unit area of wall) be light sources. different on the two walls? In which room would it be greater? 1 How did your predictions compare with your results?
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Telescopes are able to receive light that has travelled from infoBIT incredibly distant galaxies because that light has moved through the vacuum of space. There is very little matter in space to block Dark-Sky Preserve or interfere with the movement of light. On the surface of Earth, In 2004, Cypress Hills however, light is constantly being reflected or absorbed by matter. Interprovincial Park, shared by Saskatchewan and Alberta, Light Pollution was designated as a dark-sky Have you ever been on the outskirts of a city at night? If so, you preserve, a sanctuary from probably noticed an unusual glow in the sky. Light rays from street artificial light. This means it lamps and buildings travel up into the sky. There, they bounce off is a great place to view the clouds and dust particles to form this glow. Ordinarily, this light night sky and a great home for “pollution” would not be a problem, but many large telescopes nocturnal animals! The Cypress are located near big cities. Astronomers working with telescopes Hills Dark-Sky Preserve is the need a very dark sky in order to observe distant stars and galaxies. largest dark-sky preserve An international group of astronomers is now trying to get large in Canada, with 39 600 ha cities to put lampshades over their lights! That way, some of protected—that is the same these extra light rays will be directed where they are of the as 97 850 football fields! most use, to the ground.
1 A nebula is a cloud of non-luminous dust and luminous gas in space. Light from nearby stars is reflected by the dust and emitted by the gas to produce these beautiful colours. Some nebulas appear as black shadows. What would you conclude about the black shadows in these nebulas?
reSEARCH Historians use ancient records about what people observed in the night sky to help date past events more accurately. According to one historian, “Starlight is like seeing back in time.” • What do you think this means? • Find a star whose light began to travel to Earth the year you were born. • Research the Crab Nebula. Why would historians be interested in this space object?
The Orion Nebula, M42 continued ᭤
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reSEARCH 2 Describe how you could use a solar-powered calculator or other device to measure illuminance. Test your knowledge of light in space (you may need to 3 During a solar eclipse, if you are standing in the umbra do some research). portion of the shadow and look up, the Sun will be blocked and it will be very dark. If you are standing in the penumbra, 1 What is the most important the Sun will be partially blocked and you will see a crescent source of light on Earth? shape similar to a crescent moon. The diagram shows the 2 What component of the relative positions of the Sun, Moon, and Earth during a universe moves at the solar eclipse. Explain how a solar eclipse occurs with fastest speed? reference to light travelling in straight lines. 3 How long does it take for light from the Sun to reach Earth? 4 How long does it take for penumbra light from the next nearest Moon star to reach Earth? Sun
umbra Earth
Sun, Moon, and Earth forming a solar eclipse
1.3 Check Your Progress
1 Explain, using ray diagrams, how light travels.
2 How far does the light from a flashlight travel during the day? Is this different from the distance the light from a flashlight travels during the night? Explain.
3 What happens to light when it hits an opaque object?
4 What happens to illuminance as you move farther from a light source? Why?
5 Is Earth a luminous or non-luminous object? Use evidence from the photographs on the left to support your answer.
6 Gardeners and farmers know that some kinds of plants need full sunlight (high level of illuminance) while others grow best in the shade (lower level of illuminance). a) What is the light source for a plant growing in the shade? b) If you want to put a flowering plant that normally grows best in direct sunlight in a shady location, what could you do?
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BBIGIG IDEAI DEAS The law of reflection describes 2.0 how light reflects from a plane mirror.
What does the surface of this lake have in common with the Moon? What does it have in common with this page—or your face? You can see these objects because light reflects from their surfaces to your eyes. One of the best ways to learn more about this very useful property of light is to investigate a device you probably use all the time: a mirror.
Is this photograph printed upside down? How can you tell which is the reflection?
2.1 The Reflection of Light light coming in
The diagram on the right shows a design problem that needs to be solved. window • Copy the diagram into your notebook. • Think about how you could use mirrors in the design. • Draw in any mirrors you would add to this device. • How did you know where to put them? How would you solve this problem?
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Reflect on This A calm body of water can create a clear reflection. The smooth surface of the water acts like a mirror and reflects the light straight into your eyes. What happens to the reflection if you throw a rock into the water? Let us see how the surfaces of different materials reflect light and what happens when light hits a mirror.
Experiment ON YOUR OWN Light Reflection
The Question 2 Which properties of materials will you Which properties of a material can help consider? You could investigate colour, predict how well its surface reflects light? texture, shape, and/or softness. Decide if you will work with another group to divide up the tests. CAUTION! Take care when handling mirrors, as they may 3 Once you have your materials, but before have sharp edges and break easily. you test them, record their properties and your predictions.
Materials & Equipment • materials to test for reflective ability • light source • light sensor computer interface (optional) • other apparatus as required
Your Task
1 Plan how you will test your materials for their ability to reflect light. What will you Using a light sensor to test for reflection need? What variables will you control? How will you record your observations? For 4 Conduct your tests. Record your findings example, do you want to rank the materials in a table. from “most reflective” to “least reflective”? 5 If you are using a light sensor, examine How might you make measurements to the lenses in a pair of sunglasses. Record get a numerical answer (quantitative the brightness of light entering and measurements)? passing through the sunglasses.
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6 If you took measurements with a light 12 If absolutely no light is reflected from an sensor, organize the data into a graph. object, can you see the object? Why or why not? 7 Which materials did you predict would be the best reflectors? Which properties of 13 If you were going to repeat this these materials did you think would experiment, how could you improve the make them good reflectors? plan of your test? What other properties would you investigate? 8 Which material actually reflected light the best? Why did it reflect light so well? 14 When using a light sensor, what units do you use to measure the quantity of 9 Which material reflected light the least? brightness? How do these units relate to What happened to the light? Did this the illuminance? cause any change in the material itself? 15 By what percent is the brightness reduced 10 List the properties of a material that affect when you use sunglasses? how light reflects from the surface of the material. Forming Conclusions 11 Based on your findings, what kind of 16 Write your conclusion about which material would you use in each of the properties of a material help to predict following? Explain your answer. how well its surface reflects light. a) to make a mirror b) to cover hot food and keep it warm c) to make a table that will be used outdoors in a sunny location
You have been investigating how light reflects from objects. But what happens when light hits a mirror?
P ROBLEM S OLVER
Checking Out the Angles
• In a small group, focus a light source 2 Can your group arrange a group of (a flashlight, a ray box) that produces mirrors together so that the reflected a narrow ray of light so that the light light displays a geometrical shape? ray strikes a flat, or plane, mirror. 3 How can your group arrange a number 1 Discuss and formulate an answer with of mirrors so that light could be your group members: What happens reflected forever? to the light ray if it strikes the mirror at a 90° angle? Is the result different if the light strikes at another angle?
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incident ray reflected ray The Law of Reflection When a ray of light hits a mirror straight on, that is, normal at right angles, it bounces straight back. When a light ray hits the mirror at an angle to the mirror, it angle of angle of bounces back at an angle. If you use straight lines incidence reflection to represent the mirror and rays in a drawing, a line perpendicular to the mirror is called a normal. The mirror angle between the incoming ray, the incident ray, and the normal is the angle of incidence. The angle between the reflected ray and the normal is the Illustrating the law of reflection angle of reflection. How are these angles related? The relationship between these angles is the basis of the law of reflection, which you will investigate in the next activity.
I NVESTIGATOR
The Law of Reflection
The Question How does the angle of incidence compare with the angle of reflection?
Materials & Equipment • light source • pencil • plane mirror • ruler • paper • protractor • modelling clay
Procedure
1 Draw a horizontal line on a piece of paper. Use a protractor to draw a line 4 Move the light source so that the ray perpendicular to it (a normal). of light hits the mirror at an angle. 2 Using modelling clay, stand the mirror Make sure that your light strikes the upright with the silvered back along point where the normal meets the mirror. the horizontal line. What happens to the reflected ray?
3 In a darkened room, shine a ray of light 5 With your ruler, draw the incident ray at the mirror along the perpendicular and the reflected ray. Using arrows, line. What do you observe? show the direction of the light rays.
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6 Repeat the experiment using several 11 Why is it helpful to know how light different angles of incidence. reflects? How could you use this information? Keeping Records 12 How is information about light 7 Measure the angles of incidence and reflection helpful in designing and the angles of reflection on your ray creating technology for our use? diagrams using a protractor. • Use the symbol Єi to stand for the 13 Describe applications of the law of angle of incidence. reflection in everyday life. Є • Use the symbol r to stand for the 14 Pick one application and draw a ray angle of reflection. diagram labelling i) the angle of • Write your values as an equation: incidence, ii) the normal, and Є ϭ Є ϭ i ?° r ?° iii) the angle of reflection. Be sure to include arrows indicating the Analyzing and Interpreting direction of light.
8 What did you observe with respect to the angle of incidence and the Forming Conclusions
angle of reflection? 15 Write a conclusion to answer the question, “How does the angle of 9 What happens to the angle of reflection when you increase the angle of incidence compare to the angle incidence? of reflection?”
10 How does your drawing compare with the illustration on page 102?
Specular and Diffuse Reflection When bouncing a ball on a flat surface, such as your driveway, you can predict where the ball will bounce. However, if you bounce the ball on a rough surface, such as grass or a rocky path, do you know where it will bounce? Light behaves in the same A way as the ball when reflecting off smooth and rough surfaces. Reflection off smooth surfaces results in a type of reflection known as specular (or regular) reflection. Mirrors, smooth metal surfaces, or calm water will create specular reflection. If light reflects off an uneven surface such as the ocean surface, diffuse reflection occurs. A rough surface causes reflected light to scatter in many directions. B A) Specular and B) diffuse reflection
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try this at HOME Specular and Diffuse Reflection
You can study specular and diffuse reflection using aluminum foil and a flashlight. Record your findings in your Science Journal.
Procedure
1 Predict what will happen when you shine a flashlight on a smooth piece of aluminum foil. Now, predict what will happen when you shine a flashlight on a crinkled piece of foil.
2 In a dark room, shine the flashlight on the smooth, and then the crinkled, piece of aluminum foil.
3 Record your observations. Draw a ray diagram for each of the two types of reflection.
Questions
4 Did your prediction match your recorded observations?
5 What applications can you think of that Comparing the effects of a flashlight shining on smooth would use diffuse reflection? aluminum foil and a flashlight shining on crinkled aluminum foil
Useful Mirrors There are two basic ways in which people use plane mirrors.
• You can look into them and observe the reflection. Which of the photos on the next page shows this type of use?
• The second use is to take light from one source and send it in a different direction. Which of the photos on the next page shows this type of use?
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A B C
D E F
1 If you see someone in a mirror, can they see you? Explain why or why not.
2 Use a diagram to illustrate the law of reflection.
3 If light is reflecting at an angle of 27°, what is the angle of incidence? Would the angle of incidence change if the type of reflection were specular or diffuse?
4 Why do you think mirrors are a favourite tool for magicians?
5 Two rules you have learned about light are: • Light travels in straight lines. • Light reflects from a plane mirror according to the law of reflection.
Using these rules, explain why, when you look in a mirror, everything appears reversed. (Your left hand is in the place of your right hand.)
6 If you move back from a plane mirror, will you see more of yourself in the mirror? Explain why it seems that you can “see into” a mirror.
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reSEARCH 2.2 The Reflection of Light Using One technological advance that makes use of reflecting Curved Mirrors light is fibre optics. A fibre- optics cable is a thin flexible Mirrors make up many common objects. Look at these pictures. fibre surrounded by an outer covering called cladding material. Light is reflected within the fibre. • Find out how fibre optics work. • What technologies and devices use fibre optics? • Draw a ray diagram for light inside a fibre.
Mirror globe Theatre lights
• What do you think is the function of the mirror in the garden? In your Science Journal, write down your ideas on how this mirror’s unusual shape helps it perform this function.
• What do you think is the function of the mirror that forms part of a theatre spotlight? Write down your ideas on how this mirror’s shape suits its function.
What You See in Different Types of Mirrors When you look at yourself in a plane mirror, does your image depend on the distance between you and the mirror?
• Consider the size of your image. As you step back from the mirror, your image stays the same size. It appears smaller only because it is farther away.
• Consider the orientation of your image, that is, its position (compared with yourself). Your image in a plane mirror is upright, just like yourself. However, your image is reversed— your left and right sides appear as the right and left sides of Which hand is holding the comb? your image.
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Convex and Concave Mirrors When you look into a curved mirror, you see something quite different from the image produced by a plane mirror. There is a difference in the field of view of the mirrors. The geometric centre of the mirror is called the vertex and a line perpendicular to the mirror surface at the vertex is the principal axis. There are two important reference points for curved mirrors. The centre of curvature (C) is the point in space that would represent the centre of the sphere from which the mirror was cut. The principal focus (F) is a point on the principal axis at which rays parallel to the principal axis converge or appear to diverge from. The focal point is Surveillance mirror located halfway between C and the mirror. When you are shopping, you may have noticed a large mirror A in the store. Convex surveillance mirrors are used to monitor a large area. The outwardly-curved surface of a convex mirror reflects light from several parts of the room to the person’s eye so that a single mirror reflects a large area. Images in convex mirrors appear smaller than the object’s true size. Convex mirrors are also used in parking garages so drivers can see if an oncoming car is coming around the corner. School buses use convex mirrors, which allow the driver to see children in front of and beside the bus. In addition to convex mirrors, there are concave mirrors, ones in which the reflecting surface is curved inward like a bowl. Concave mirrors are used to B focus light rays or to create larger images. Common concave mirrors are those found in car headlights or those used to apply makeup. In the next activity, you will investigate the size and orientation of images in convex and concave mirrors.
A
oobjectbj imagee FC There is a difference in the field of view of the A) plane and B) convex mirrors.
B C
object A) Convex mirror ray diagram. An image image object F formed in a convex mirror is upright and smaller than the object. B) and C) concave C image FC mirror ray diagrams. The position, size, and type of the image depends on how close the object is to the mirror.
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P ROBLEM S OLVER
Images in Concave and Convex Mirrors
How do you think the image of an object in a concave mirror is different from its image in a plane mirror? from its image in a convex mirror? To find out, follow these steps: • Use a concave and a convex mirror and any object (your finger, a pencil, etc.). • Position the object in front of the mirror so the image in the mirror is as clear as possible (in focus). • Compare the size and orientation (upright or upside down) of the image with the actual object. • Predict what will happen to the size and orientation of the image when 1 Test your predictions and record your the object is placed at different observations in your Science Journal. distances from the mirror. 2 What happened to the image in the concave mirror as you moved the object very close to the mirror? far away from the mirror?
Characteristics of an Image in a Curved Mirror Curved mirrors produce images different from those that a plane mirror produces. When you are looking at a plane mirror, your image appears behind the mirror. This image is known as a virtual image. Light does not pass through the mirror, it only Real image formed by looking at a appears that light is travelling from this location. Curved mirrors concave mirror from far away. can produce both virtual and real images. Real images are created on the same side of a concave mirror as the object; light passes through the image location and the image formed can be caught on a screen. When the object is far away from a concave mirror, the image is upside down and smaller, but is a real image. When the object is very near, the image is upright and larger than the object. The enlarged images formed from concave mirrors when the object is very close are virtual images.
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When the object is far away from a convex mirror, the image is upright and very small. As the object comes closer to a convex mirror, the image remains upright and becomes larger, but is still smaller than the object. Convex mirrors produce only virtual images.
Virtual image formed by a Uses of Concave and Convex Mirrors convex mirror There are many applications of concave and convex mirrors. The table below shows some of the common applications.
Concave Mirrors Convex Mirrors Device Use of Concave Mirror Device Use of Convex Mirror Flashlight • to produce a parallel beam of Security mirror • to increase the field of view light leaving the flashlight allowing all parts of a room to be monitored
Telescope • to collect a large amount of light Side vehicle mirror • to view a large area beside and from a star or other distant behind the vehicle, reducing source and focus it for viewing blind spots
Cosmetic or shaving mirror • to produce an enlarged image Safety mirror • to reflect every angle you need to of the face see, thus decreasing dangerous collisions and accidents
Headlights of a car • to produce a parallel beam of Mouth mirrors or • to allow indirect vision and light leaving the car that can “dental mirrors” lighting in confined spaces; be pointed down (low beam) used by dentists and engineers or straight ahead (high beam)
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1 What would happen to the reflected light if you shone a flashlight onto: a) the convex side of a metal bowl? b) the concave side of a spoon?
2 Use a labelled ray diagram to illustrate the “law of reflection” in a plane mirror.
3 A baby can sometimes be fooled into believing a mirror image is the real thing. A young child rarely can. What clues help children learn to tell a mirror image from the real thing?
2.3 Check Your Progress
1 How does the reflection of light help explain why you can see non-luminous objects?
2 What types of materials reflect light best?
3 Draw a capital “B” about 2 cm tall at the top of a piece of paper. Below it, use ray diagrams to draw your predictions of how this letter’s image would appear in: a) a plane mirror b) a second plane mirror placed at an angle to the first plane mirror (that is, the image of the image of the letter) c) a concave mirror, with the letter near the mirror d) a concave mirror, with the letter very far away from the mirror e) a convex mirror
4 Why do car side mirrors that have a convex mirror attached to them carry a warning to drivers that says, “Objects seen in the mirror are closer than they appear”?
5 The circled objects in the picture are, or contain, mirrors. a) What type of mirror is being shown in each case? b) How are the reflections in each mirror of use to the bus driver? c) Could a different type of mirror be used in each case? Explain why or why not.
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BBIGIG IDEAI DEAS Light is refracted by transparent materials, 3.0 and this is what makes lenses so useful.
You have learned how mirrors take advantage of one property of light: reflection. The law of reflection tells us that we can use materials that reflect to collect light and change its path. What other properties of light can be useful? How about its speed? Light travels at about 300 000 km/s in a vacuum such as space (299 792.5 km/s to be more exact). This means that it takes light from the Sun about 8 min to reach the surface of Earth. We use the speed of light to measure very large distances in space. Refer to the diagram below.
Travelling in Space Is the speed of light always the same? No, because light slows down when passing through water, air, or other materials. In glass, for example, light travels at a mere 195 000 km/s. That is about 65 percent of the speed at which it travels in the emptiness (or vacuum) of space. What happens to light when it suddenly changes from a speed of 300 000 km/s to 195 000 km/s?
It takes 8 min for light from the Sun to reach Earth. A space probe can travel from Earth to the Sun in 25 days.
It would take 72 years for a race car to travel the distance from Earth to the Sun.
If a jet could travel at the speed of light, it would go around the equator seven times in 1 s!
Compare the speed of light to other moving forms.
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3.1 Refraction
Have you ever tried to pick up a pebble or catch a tadpole in your hands when wading in a lake? Think about what that experience was like. Objects underwater look as though they are actually slightly nearer than they really are. Why? What is happening to the light as it reflects from an object underwater to your eye? Look for a clue in the photograph on this page. Write down your ideas in your Science Journal.
Is this object really bent? Skillful Spearfishing How do you know? In North America, Indigenous groups who have traditionally hunted fish with spears have developed skills to solve the problem of underwater objects looking nearer than they really are. In Alaska, Indigenous people fish with spears as did the Inuit and the First Nations peoples of northern Saskatchewan. The native people of Alaska spear fish in the fall when the river begins to ice up. They hunt from a boat on the river or from shore when the ice is thick enough to support a hunter’s weight. Fishing is done at night using a lantern, which is shielded on one side so that only a fraction of the light is emitted. The men spear fish from the bow of the boat as ice rushes across the surface of the water. In the darkness no shadows are cast and so the fish are not alerted to the presence of the hunter. On the shore ice, the hunters cut a wide hole. Young spruce trees are peeled and lashed together. This framework of spruce trunks is sunken with rocks below the hole cut in the ice. The Successful spearfishing requires peeled spruce appears white against the sand and silt of the many skills. riverbed. When fish swim over the framework or bump into it, their presence is obvious and they can be speared. Spearfishing is made more difficult because of refraction. Refraction occurs when light passes through one substance into another. As light passes from the water into the air, the light changes direction. This change in the direction that the light travels causes the fish to appear where we think we see it, but in fact the fish is really lower in the water. Refraction causes the optical illusion so that the fish appears larger and closer to the surface than it really is. When a hunter attempts to spear the fish Because of refraction the fish from directly above, refraction does not affect the image of the appears closer to the surface fish. It appears to the hunter where it actually is. Hunters must than it really is. understand how to deal with optical illusions.
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Pretend Fishing
Different fluids have different thicknesses • With a toothpick, test to see how easy it and pour out of containers at different is to spear the jelly bean in each fluid. rates. Do you think that different fluids 1 What differences do you notice? will affect light differently? Try to answer this question. 2 What happens to the toothpick as it • Fill a clear drinking cup with oil and enters the water? the oil? another with water. • Place a jelly bean or a gummy bear in each cup.
Changes in Direction The change in direction of light, called refraction, occurs because light travels at different speeds through different materials. As light passes from air to A B C D E water, it slows down and changes smooth direction. How does this happen? It happens because light travels forward much like the line of skaters shown rough here. If some of the skaters at the end of the line slow down on rough ice, but the rest continue at the original speed, the line of skaters will change direction. When part of a beam of light slows down, Skater E slows down, making the entire row of skaters turn. Compare this and the rest keeps going, the result is situation with the pencil in the glass of water on the previous page. a change in direction of the light.
Refraction Through a Liquid As discussed previously, hunters who have grasped how to deal with optical illusions in water miss fewer fish. Sea hawks and fish eagles must learn this lesson too. They must compensate for the refraction of light. If they do not, they will miss many meals. To further explore refraction, you will investigate the refraction of light through various substances.
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Refraction in Water and Other Liquids
The Question a) Cut two parallel What happens to the path of light rays when slits in the they pass from air into water or other liquids? cardboard approximately 2 cm apart, as Homemade ray box Materials & Equipment shown in the picture. Each slit should measure • piece of white paper about 5 cm high by 0.5 cm wide. • tape b) Use modelling clay to stand the • pencil and ruler cardboard upright in front of your • ray box with 2-slit opening, or other light source. light source—(see alternative below) 2 Darken the room and move the flashlight • 250-mL beaker or jar in front of the cardboard (or move the • water ray box) until the two rays of light • other liquids to be tested coming through the slits are parallel. If a ray box is not available: • piece of cardboard, measuring 10 cm by 15 cm • scissors Step 2 • modelling clay to support the cardboard 3 Tape the piece of paper on the table in front of the cardboard or the ray box. Use a pencil and ruler to mark a line perpendicular to the cardboard or the front of the ray box starting between the two slits and extending outward. This is the normal. Now, mark the path of the two rays of light.
Materials and equipment
Procedure
1 If you do not have a ray box that can produce two parallel rays of light using a 2-slit opening, make your own apparatus as follows: Drawing the normal
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4 Read through the rest of the activity d) When you test each liquid, use a and make a table in which to record different colour of pencil, or a new your observations. piece of paper, to mark any change in the path of light. 5 Put the empty beaker or jar in the path of both rays of light. What do you observe? Keeping Records 6 Fill the beaker with water and place it 9 How could you use a graph to compare so that both rays of light pass through changes in refraction you observed? it. What do you observe? For example, on the vertical axis you
7 Move the beaker of water until the rays could put the distance from the beaker of light leaving the beaker are as to the point where the refracted rays focussed (clear) as possible. came together. What will you put on a) On the piece of paper, mark the the horizontal axis? What type of graph path taken by the rays of light should you use? Draw your graph. leaving the beaker (refracted rays). What do you observe? Analyzing and Interpreting b) Record the distance from the cardboard 10 How did the water affect the path taken or the ray box to the beaker. This is by the rays of light? How did you know where you will put other beakers this change was due to the water and of liquid during your tests. not to the glass of the beaker or jar? 8 Repeat steps 6 and 7 using other liquids, 11 When you tested different liquids, as follows: which factors made the most difference a) When you select the liquids to test, in how much the light was refracted? think about what factors might affect how light passes through them. 12 Compare your predicted path and the You may wish to tested path. Was your predicted angle – compare clear liquids to more of refraction greater than or less than opaque (cloudy) ones. the tested angle of refraction? – test if thicker, more slowly pouring, 13 Write your own definition of refraction. liquids such as oils cause light to behave differently from thinner 14 What factors affect refraction the most? liquids such as water. Why do you think this is so? b) Are there visible particles present in the liquid? Record these factors Forming Conclusions
before making your tests. 15 Write a conclusion about what happens c) Predict the path of the refracted rays to the path of light rays when they pass by drawing a dashed line on the from air into other liquids. How does paper. Use the same colour of pencil your evidence support or refute the to draw a solid line when you test predictions you made about the refraction the liquid in the next step. of light through the different liquids?
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Refraction Through Glass Glass is transparent and easy to shape. Mirrors are made by applying a reflective coating to glass. Is there some property of glass that would make it useful for refraction as well? What happens when light travels from air into glass? To find out, try this activity.
I NVESTIGATOR
From Air to Glass
The Question 2 Darken the room and adjust the ray box What happens to light when it passes from so the edges of the ray are parallel. air into glass? 3 Direct a ray of light so that it shines along the line perpendicular to the block. Trace the ray as it enters; this is Materials & Equipment called the incident ray. The point • ray box with a single-slit opening or other where the ray enters the block is called light source—(see alternative on p. 114) the point of incidence. • glass block • ruler 4 Observe the ray through the glass block. • paper • protractor On the paper, mark the point where the ray leaves the block. The line joining the Procedure points where the ray enters and leaves the block will trace the path of the ray 1 Place the glass block on a piece of in the glass. This is the refracted ray. paper and trace around it. Mark a point near the middle of the front edge of the block. Draw a line on the paper perpendicular to the edge at this point. (As in the case of reflection in mirrors, this line is also called the normal.)
Step 4
5 Move the ray box so that the light strikes the block at the same point of incidence, Step 1 but at an angle from the normal.
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6 Trace the incident ray. Mark the point Analyzing and Interpreting where it exits the block. Give the incident 10 How did the incident ray compare ray and exit point matching labels. with the refracted ray? 7 Repeat the last two steps using 11 What happened to the light when it different angles for the incident ray. entered the block at a 90° angle? Then, remove the glass block. What happened when it entered at a different angle? Keeping Records 12 What happened to the refracted ray 8 Complete each refracted ray on the when the angle of incidence was paper by drawing a line from the increased? point of incidence to each exit point. Carefully label each refracted ray to match the incident ray. Add arrows Forming Conclusions to indicate the direction of the rays. 13 What two factors affect how much light is refracted? 9 Determine the angles of incidence and refraction in each case. These angles 14 Based on your findings, where do are measured from the normal, as with you think you should stand to look reflection in mirrors. through the glass of a window?
When light travels through an interface between two regions at an Refractive indexes of angle other than 90°, it will most likely change direction depending some common substances on the properties of each region. For example, when light travels from Substance Refractive air to water, it will change direction toward the normal. When light index travels from water to air it will change direction away from the normal. air 1.003 This change in direction happens if light travels at different speeds on ice 1.31 either side of the interface. For example, light travels at different speeds through glass and water. The refractive index (or index of refraction) water 1.33 is a measure of how much the speed of light is reduced in the region. crown glass 1.42 glycerin 1.47 normal, at a right angle to the surface ruby 1.54 incident ray lower refractive index (e.g., air) diamond 2.42 i
r higher refractive index i (e.g., glass or water)
r Refraction of light is caused by light passing through two regions whose refractive indexes i = angle of incidence refracted ray r = angle of refraction are different.
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Heat Waves
During a hot summer day, why does the • Set up the apparatus shown in the highway look wet and the surface diagram. shimmer in the distance? • Focus the light on the screen 3–4 m Light is refracted as it passes through from the projector. different forms of air. In summer, as light passes through hot and cold air over the • Light the Bunsen burner and place highway, a reflection of the sky occurs it 50 cm in front of the screen. and the shimmering and wet appearance of the road is created. This is called a mirage. CAUTION! Take precautions when working with flames.
• Sketch the patterns in your Science Journal.
1 Have you seen these patterns before? If so, describe where you saw them. Heat-wave apparatus
Making Use of Refraction You have learned that light is refracted (changes direction) as it passes through different materials. This refraction depends on factors such as the angle of incidence and the type of material. What clues do you have about how this property of light could be useful in practical applications? In your Science Journal, write down your ideas about how optical devices might use refraction. • Think about the shape of the mirrors you investigated in the previous section. • Predict how shape might affect refraction through glass.
1 When would it be easier for a bear to catch a fish: as the fish jumps into the air, or as it swims? Explain your reasoning in terms of how light travels through air and through water.
2 Analogies can help people communicate ideas. The refraction of light was explained using the analogy of a synchronized skating team. Suggest another analogy to help explain to someone else the refraction of light as it passes through different materials and slows down.
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3 Why do objects at the bottom of an aquarium filled with infoBIT water look closer than they really are? Mirages 4 Imagine that you have a waterproof flashlight, underwater In this picture, light is playing a in a swimming pool. You point the flashlight perpendicular trick on you! What looks like a (at 90°) to the surface of the water. Predict how the path of light lake is not really there at all— would change direction as it came out of the water into the it is a mirage. Mirages happen air. What would happen to the light if the flashlight was when layers of air with different pointed at 45°? temperatures lie on top of each other. As light passes through one layer of air into the next, it 3.2 Using Refraction to Focus Light refracts, or changes direction. This can create imaginary pools The people below are all using the property of refraction to of water, upside-down images, accomplish something. What do you think that is? Think back to and even castles in the clouds! the activity in which you shone two parallel rays of light through So, the next time you are water. What happened as the rays left the water? How might this riding in a car, don’t worry that be related to what these people are doing? the road ahead looks wet even though it is not raining. You are just looking at a “wet road” mirage. Light from the sky changed direction in the hot air over the road, creating the illusion.
Is this water on the road or a mirage? Looking Through Curves When you experimented with mirrors, you found that the shape of the mirror had an effect on the image. For example, the image reflected by a concave mirror was smaller and upside down when the object was far away. The image reflected by a convex mirror was larger and right side up. These characteristics of curved mirrors make them very useful. In the same way, the shape of a block of glass through which light passes has an effect on the refracted image. A lens is a piece of transparent material (glass, plastic) with at least one curved surface. In the activity on the next page, you will explore the effect of different lenses on light.
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Looking at Convex and Concave Lenses
There are two main types of lenses: • When you can see the graph paper convex and concave. What do you think clearly, measure the distance from the happens to light as it moves through each? lens to the paper at intervals as you How would you be able to tell? move the slide upward. To determine what happens to light – Each time, record the apparent size passing through a lens, use a depression of the graph area seen through the (well) slide, a piece of graph paper, and lens by counting the number of a ruler as follows: squares you see. • Choose an area on the graph paper that just fits into the depression on the slide. Measure and record the area as the “number of complete squares” you can see.
• Hold the slide so that you are looking through the convex surface. Place it over the graph paper.
• What happens when you move the convex lens away from the paper?
• Repeat this procedure using the concave surface.
• Plot a graph of the number of complete squares that were visible in relation to the distance from the lens for both lenses.
1 What relationship did you discover?
2 What do you conclude about convex and concave lenses?
A depression slide, or well slide, contains a curved area that you can use as a lens. The slide is acting as a convex lens.
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Types of Lenses A convex lens curves outward. You can quickly spot a convex lens by noticing it is thicker in the middle than at the edges. A concave lens curves inward. You can tell because concave lenses are thicker at the edges than in the middle. Look around your classroom or home and see how many lenses you can discover. In your Science Journal, make a list noting some of the features of each lens: shape, thickness, whether it magnifies, whether it appears to be double or single, whether it is made of glass or plastic, and any other features you think are interesting. In the next activity, you will look more closely at how light is refracted through convex and concave lenses.
Simple convex Double convex Simple concave Double concave
I NVESTIGATOR
Refraction of Light Using Lenses
Before You Start The Question In this activity, you are going to observe How does light refract as it passes through how light passes through two types of convex and concave lenses? lenses: convex and concave. Think about what you have learned about reflection and refraction. In your Science Journal, Materials & Equipment write a prediction for how light might pass through each type of lens. • convex lens • concave lens • ray box with both a 3- and a 5-slit opening • paper • pencil • ruler
What happens to light when it passes through a lens? continued ᭤
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Procedure 8 What shape is a magnifying lens? Which of your lenses could enlarge objects? 1 Set up the ray box so that the edges of How did the other lens compare with the beam are parallel and then insert respect to magnification of objects? the slit opening. Darken the room.
2 Shine the light rays at the convex lens so that the middle ray goes through the centre of the lens. Observe what happens to the light.
3 Repeat step 2 using the concave lens. convex lens
Keeping Records
4 Record your observations in your Science Journal. Draw a diagram noting the incident rays and the refracted rays for both lenses. concave lens A convex lens causes light rays to come together, or to Analyzing and Interpreting converge. A concave lens causes light rays to spread out, or to diverge. Copy and complete these diagrams to show 5 How do the shapes of the two lenses the rays after passing through the lenses. differ?
6 How did your predictions of the Forming Conclusions refraction of light for each lens 9 If all the factors are the same except compare with your observations? for the type of lens, what influences the 7 Compare the refraction of light through way in which light bends as it passes a convex lens and a concave lens. through a lens?
1 Many stores sell “reading glasses” to help people see the very small print in newspapers and other publications. a) What type of lenses would these glasses contain? Explain. b) Would these glasses help people see more clearly at all times? Why or why not?
2 This picture shows the glass bricks builders use to make decorative walls in homes, offices, and stores. How do these bricks preserve privacy while allowing light to pass through?
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3 Explain the difference between the ways that a concave lens and a convex lens refract light.
4 A very large aquarium in a zoo has areas where the glass wall is curved outward. Describe the view a fish would have of a person who is looking into the tank through one of these curved areas.
3.3 Check Your Progress
1 What factors affect how light is refracted through a material?
2 A “magic” trick involves putting a coin in the bottom of a cup, then having the person step back until he or she can no longer see the coin from the top. The magician then makes the coin “reappear” by pouring in water. How does this trick work?
3 Some light fixtures contain a lens through which the light passes before it is used. In each of the following situations, decide whether this lens should be convex or concave. Use a diagram to explain. a) You are designing an outdoor light that will spread a soft light over a large area. b) You are designing a reading light that someone could use without the light bothering others in the room. c) You are designing a store light that will keep the floor well lit, but which will also shine most of its light on the display case directly below.
4 Explain why the newsprint in the drops of water is magnified in the photograph.
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BBIGIG IDEAI DEAS The properties of light explain how 4.0 the eye and the camera capture images.
“Buffalo Bulls Fighting” on the North Saskatchewan River (1848–1852) by Paul Kane
Early explorers either were artists themselves or brought artists with them in order to bring home images of what they saw. Their maps and paintings excited the imaginations of many, helping to encourage further exploration of the world. How do you record images of what you have seen or experienced? Today, artists still sketch and paint, but a far more common technology used by many people is the camera. Whether you use an instant camera, a video camera, a digital camera, or take pictures through your cellphone, all the technologies rely on the same basic principles of operation as your eye.
At one time, cameras used only film to record an image. Now, they also use videotape and digital media.
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4.1 How Does Light Enter Your Eye?
Make a two-column table in your Science Journal. Title the first column, “Parts of the Eye.” List all of the parts of your eye that you know, leaving space to add more. Title the second column, “Function.” Then, do the following activities while observing a partner’s eyes or using a mirror. Record your observations in the “Function” column. • Blink. What happens? Which parts of the eye are involved? How do you know? • Hold your head still. Move your eyes from side to side, and up and down. Which parts of the eye move? • Put one hand over your left eye and keep it there for 30 s. Remove the hand and observe your eyes, comparing the left eye with the right eye. What happened? Why do you think this is so? Which parts of the eye were involved? • If time and weather permit, stand outside for a few moments. Observe the appearance of your eyes or your partner’s eyes. Then, go back inside the building and immediately observe what happens. • Look at something far away for a moment, then look immediately at something very close. What did you notice? Observing functions of the eye Once you have completed as much of your table as you can, put question marks beside any parts of the eye whose function you could not figure out. Write down any questions you have about how the eye works.
The Hole to the World You have learned that light either travels from a source to your eyes or reflects off an object to your eyes. But how exactly does light enter your eye? Both the eye and the camera on the next page have a hole that lets in light. • In the eye, this hole is called the pupil. • In the camera, it is called the aperture. The pupil of your eye is surrounded by a band of muscle, called the iris. This band controls the size of the pupil, and so controls the amount of light that can enter your eye. In dim light, the iris opens and the pupil dilates, or becomes wider, so you can gather more light (see photos on the next page). In bright light, such as outside, your iris closes down so the eye receives just the right amount of light. This happens automatically, without your conscious control.
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In the same way, the diaphragm changes the size of the aperture of a camera lens to allow in the proper amount of light. The shutter of a camera acts like a door. If the shutter is open for a long time, more light enters the camera. Which part of your eye is like a camera’s shutter?
The pupil is not dilated. The aperture is very small.
The pupil is dilated. The aperture is wide open.
What Happens Inside In order for you to see, something must react to the light entering your eye. The part that accomplishes this for you is the retina, a special lining on the back of your eye. When light hits the retina, receptor cells send messages to the brain, which are translated into an image. In a similar way, the light entering a digital camera hits the charged coupled device (CCD) cell. Several cells are combined to form the CCD matrix, which has a similar function to the retina, because it contains photosensitive elements. When light reaches the photosensitive elements in the CCD matrix, pixels are illuminated. Each pixel represents one portion or location in the image. Red, blue, and green colour filters over the individual pixels let the appropriate light through. The illumination of the pixel creates an electronic pulse and the built-in computer in the camera creates digital data. This essentially means that light is turned into electricity. Digital cameras with a higher pixel rating (a larger number of sample locations in each area) produce larger and more refined pictures.
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When working with mirrors and lenses, you discovered that an image is only clear and distinct when it is focussed. Another way of saying this is that the light rays forming the image converge, or come together, at one point, called the focal point. Your eye has a transparent lens, too, just behind the pupil. This lens focusses light on the retina so you can see a clear image. The lens of your eye automatically changes its shape to focus on near or distant objects. In the same way, a camera has one or more lenses to focus incoming light on the CCD matrix. Some cameras focus automatically while others require the user to manually focus the lens.
lens retina pupil
light rays entering eye
The image formed on the retina is upside iris down. You do not see things that way because your brain interprets the information sent by the retina and flips it around.
light rays entering camera upside-down image recorded on the CCD matrix
lens aperture reSEARCH Myopia and hyperopia can be The image formed on the CCD matrix is also upside down. However, the camera software corrected using lenses. Other displays the photo correctly on the camera’s screen. problems of human vision exist. Research one of the problems listed below. Prepare a visual Solving Eye Problems with Lenses presentation and ray diagram What happens when you look through a lens? Light is refracted of the eye illustrating the by that lens, then is refracted by your eye. The combination lets problem and how it can you see a magnified image. There are many ways lenses are used be corrected. in combinations. Perhaps the most important in everyday life • presbyopia is to help correct vision problems. If you examine a pair of eyeglasses, you will notice that they are made up of two lenses. • astigmatism Which type are they: concave or convex? • keratoconus
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Many people wear glasses or contact lenses because their eyes do not focus images on their retinas properly. The additional lenses refract the light entering the eye so that the lens in the eye will produce a good, clear image. The two most common problems, myopia (near-sightedness) and hyperopia (far-sightedness), are shown below.
retina
In a near-sighted person, the eyeball normal vision may be too long or the lens too thick. In this case, light entering the eye is focussed too soon, short of the retina. When a near-sighted person tries to see things at a distance, the image is blurred. In a far-sighted near-sightedness (myopia) near-sightedness corrected by lens person, the eyeball may be too short or the lens too thin. In this case, the refracted light focusses behind the retina. Objects close to a far-sighted person will be blurred. Lenses can be used to correct both of these problems. far-sightedness (hyperopia) far-sightedness corrected by lens
Many Eyes Not all animals see the world in the same way that humans do. Snakes see adequately. However, they are able to see motion better than still objects and have been known to leave motionless prey unharmed during the day. Some snakes such as rattlesnakes have sensitive receptors in grooves between their nose and eyes, allowing them to pick up heat signals from warm objects. Horses and similar animals, such as zebras, have excellent peripheral vision due to the placement of their eyes. Having eyes on the side of the head allows these animals to see advancing predators from many directions; however, they have a blind spot directly in front of their noses. Deer also have excellent peripheral vision due to the size of their eyes, providing a large field of view. Deer can see only the colours blue and green. Since they cannot perceive any difference between red and green, they are essentially red-green colour-blind, as some humans are. Dogs and cats are also colour blind. However, dogs and cats have better peripheral and night vision that humans do. Human eyes have a filter that blocks almost all ultraviolet (UV) light. Deer eyes do not filter UV so they are sensitive to the UV spectrum. You will learn about UV and the electromagnetic spectrum in Big Idea 6.0.
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Most insects have compound eyes. Compound eyes may have hundreds to thousands of sections, depending on the insect. Dragonflies have the largest number of sections in their compound eyes. They are very efficient hunters, catching their prey in flight. Grasshoppers have three simple eyes and two large compound eyes. They have fewer sections in their compound eyes and see grainy images compared to those that dragonflies see. Honeybees’ compound eyes cannot see the colour red; however, they can see regions of the electromagnetic spectrum that humans cannot see. A dragonfly eye displays the large The Eagle — Significant Sight number of sections in the compound eye. In First Nations cultures, the Eagle is very important. Some believe that the Eagle can see far into the past and far into the future; that it flies highest in the North American sky; and that it carries prayers to the spirit world. The Eagle represents compassion, respect, detachment, and humility. These qualities are also the qualities of an Elder. • Many First Nations give an Eagle feather as a great honour to any worthy member of their nation who has contributed in The Eagle flies to great heights and an outstanding way as a warrior, a teacher, or a person who is can see many things below. revered as a leader or Elder. The individual receiving the feather must understand that as there are two sides to the structure of the feather, so there are two ways of using the feather. On the one side there is authority; on the other, compassion. • Eagle feathers are used at powwows by many First Nations peoples. They make up the Eagle staff, which is analogous infoBIT to the Canadian flag at these gatherings. • Eagle feathers are harvested for ceremonial purposes only. Can Cats See in the Dark? Those people who have earned the right to collect Eagle You may have heard that cats feathers must have permission from their Elders and and other animals, such as communities, live in the traditional way, have direction and owls, can see in the dark. visions from the spirit world, and undergo ceremonies such However, no animal can see as fasts before they undertake the harvest of these feathers. in the absence of light. Cats Modern people sometimes try to buy Eagle head-dresses, and owls have very large pupils, but this is not appropriate. Feathers must be freely given which allow their eyes to take for outstanding contributions to the community. in as much light as possible. They also have a layer inside
1 Return to your table that lists the parts of the eye and their their eyes that acts like a mirror function. Add any new information you have learned. to reflect light inside the eye. Make a similar table listing the parts of a camera and their This also helps them see function. How similar is the camera to a human eye? quite well in dim light. Why do you think this is so? How are they different?
continued ᭤
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infoBIT 2 Explain the following observations about a camera, based on what you know of its function and how light travels. Snow Goggles a) The inside of a camera is usually black. Inuit hunters developed b) To take a picture in dim light, you must open the technology to limit the glare aperture wider. of the Sun’s light reflecting c) To take a picture in very dim light, you can leave the off the spring snow. Snow shutter open for a period of time. goggles, similar to those seen 3 Looking directly into a very bright light can damage the in the movie Atanarjuat the retina. Even a brief exposure can cause discomfort and leave Fast Runner, were the first bright after-images that make it hard to see. When there is sunglasses. The goggles were a solar eclipse, most of the Sun’s light is blocked for a few carved mainly from caribou seconds. Knowing that the iris opens to dilate the pupil antler, whalebone, or ivory; in dim light, why do you think it is so dangerous to stare driftwood was also used. up at an eclipse? Narrow slits in the snow goggles reduced the amount 4 Make a table to compare dragonfly, deer, and human vision. of light, protecting the hunter’s eyes and preventing snow blindness. This also 4.2 Other Optical Instruments improved visibility.
Human beings have remarkable eyesight. You can read this page, then look up to see who might be coming in the classroom door. You can look out a window at the sky and spot a plane flying overhead. The next moment, you could count the tiny hairs on one finger. Being human also means being curious. It hasn’t been enough to simply duplicate the way the human eye works and record what we see with cameras. People have wanted to see more, so they have designed optical instruments to extend human vision. In your Science Journal, record five things you would like to be able to observe that you can’t see with your eyes alone.
Seeing Things Up Close or Far Away You now have an understanding of how light can be reflected using mirrors and refracted using lenses. However, lenses and mirrors can be used in combination to create optical instruments. On the next page is a time line of the development of optical instruments from the days of the Romans up to today.
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infoBIT Optics Time Line
Emerald CE 54–68 54–68: Nero, Emperor of Early Eyeglasses Rome, is reported to have About 1300: The wearing of watched events in the eyeglasses becomes common. coliseum through an emerald that may have acted like a simple lens or sunglass.
CE 1300 Telescope Early Compound 1608: Dutch optician Microscope Hans Lippershey is About 1590–1609: Dutch generally credited scientists develop the with inventing the first compound (multiple telescope (a lens) microscope. refractor). It was later refined by Italian astronomer CE 1590–1609 Galileo Galilei. CE 1608 Contact Lens Bifocals CE 1784 Intraocular Lens (IOL) Kodak Camera 1949: The first plastic lens is successfully Hubble Space CE 1887 implanted into the Telescope (HST) CE 1888 human eye by British 1990: The HST is launched surgeon Harold Ridley into orbit. It is the first at the Thomas Hospital telescope to view space in London. from outside Earth’s atmosphere. CE 1949 James Webb Space CE 1990 Telescope (JWST) The Keck Telescope CE 1992 2014: The JWST is 1992: The Keck Telescope scheduled for launch has a primary mirror CE 2014 in 2014. The JWST is of a unique design. a large space infrared The 10-m diameter telescope and is expected mirror is composed of to find the first galaxies 36 hexagonal segments. that formed in the early universe.
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Common Optical Devices
Telescopes Telescopes also use lenses and mirrors. There are three types of telescopes: refracting telescopes, which use convex lenses to change the direction of light; reflecting telescopes, which use mirrors to collect light and reflect it to form an image; and telescopes that include parts of both reflecting and refracting telescopes. The world’s largest telescopes are reflecting telescopes.
primary light-gathering lens eyepiece lens
Refracting telescope
Internal cross section of a refracting telescope
eyepiece primary light-gathering mirror lens focus
secondary mirror
Internal cross section of a reflecting telescope Reflecting telescope The objective lens or mirror of a telescope gathers the light. The greater its diameter, the more light it can collect. More light means more information from a planet or star. The light forms an image, which can be magnified further by the eyepiece lens. In some telescopes, the image formed by the objective lens can be used to expose film or to activate a CCD matrix. As mentioned before in the discussion of a digital camera, this matrix contains a computer chip that turns the light into a digital signal. Several factors can make it difficult to achieve a sharp image with a telescope, including the steadiness of the object and the alignment of the object and the telescope. The stability of the optical system depends on: • the clarity of the sky • the balance of the telescope (must be able to stay in position) • the ease with which the telescope can be adjusted
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Although telescopes are useful for looking at the stars, they are not very easy to carry around. People who wish to look at other less distant objects often use binoculars. Binoculars are really two short refracting telescopes fixed together. movable eyepiece focussing wheel
fixed objective lens eyepiece
prisms Binoculars are useful at sporting The Microscope events, music concerts, live theatre, You see the greatest detail when you look at an object about or when observing nature. 25 cm from your eye. If you need to see more detail, you use a microscope. Microscopes have at least two lenses—the objective lens and the eyepiece lens—as well as a light source. The light passing through the specimen is focussed by the objective lens to form a magnified image. The eyepiece lens magnifies this image so that you see an even larger image. A microscope may have several objective lenses, each magnifying the specimen a different amount. The most powerful light microscope can magnify up to 2000 times.
A monocular A cross section of microscope the microscope has just one eyepiece lens.
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T ECHNOLOGICAL P ROBLEM S OLVER
Magnify It!
The Need 5 Build a prototype of the devices from Optical devices use a combination of the materials decided on by your group lenses and possibly mirrors to magnify during the brainstorming session. and focus an image. Your task is to design two magnifying devices. The first device Test and Evaluate magnifies distant objects, and the second 6 Test your devices to magnify distant device magnifies objects that are near. and near objects. Did they meet the design criteria set by your group? The Criteria Record your observations. • both devices must be able to magnify and focus images CAUTION! • one device is able to magnify distant Don’t look directly at the Sun with any objects magnifying instrument! • the second device is able to magnify close objects 7 If needed, adjust your designs and resketch the ray diagrams. Retest your Materials & Equipment devices after the adjustments so that your devices are operational. • 2 convex lenses with a large curve • 2 convex lenses with a small curve • decide what materials, other than lenses, you will need to make your optical devices
Brainstorm
1 In your group, plan how the lenses and other items, such as mirrors, should be arranged to allow you to: a) magnify distant objects or objects behind you (a telescope) b) magnify near objects (a microscope)
2 How will you focus your optical devices? Communicate 3 List additional items you will need to construct your magnifying devices. 8 What did making and using these magnifying devices teach you about Build the differences between microscopes 4 Sketch two ray diagrams of your and telescopes? magnifying devices. Label the 9 Demonstrate and explain the group objective lens and eyepiece lens. devices to the class.
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1 a) Which property of light do mirrors demonstrate? b) Which property of light do lenses demonstrate? c) How are mirrors and lenses useful in creating optical instruments?
2 You are a member of a group deciding where to build a new telescope. You have considered several locations (listed below), each with at least one advantage. Which location will you recommend? Why? a) in the city centre, close to the university and other schools b) near the local airport, convenient for visiting astronomers c) on the top of a tall mountain, where the atmosphere is thinner d) on an ocean-going vessel, so the telescope can be moved to different locations to view different parts of the sky
4.3 Check Your Progress Column A Column B retina one or more lenses 1 Copy the table to the right into your notebook. Column A lists the parts of the eye. Column B lists pupil and iris aperture the parts of a camera. Match each part of the eye eyelid CCD Matrix with the part of a camera that performs the same or a lens shutter similar function. Describe that function for each pair.
2 Brainstorm as many optical instruments as you can that use lenses and mirrors to control light. Choose one. Based on your experiences and observations, use a diagram to describe how the lenses and mirrors may be arranged in the instrument. Show how light is refracted or reflected and indicate the point of incidence, incident ray(s), and refracted or reflected rays. Show the direction of travel of light using arrows.
3 Which instrument was used to view each of the objects shown here? a) magnifying glass b) microscope c) telescope d) binoculars A B e) unaided eye
C D E
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Careers and Profiles Mark Duffy: Photographer
Mark Duffy is a nature, wildlife, landscape, and portrait photographer. He has achieved the Craftsman of Photographic Arts and Master of Photographic Arts from the Professional Photographers of Canada. He is part owner of McMaster Studios in Moose Jaw, Saskatchewan.
Q: How did you get into photography? A: My first full-time job was in 1976, in Edmonton, working with and developing microfilm. I then moved to Yellowknife and worked with the Yellowknifer newspaper. Later, after some time in Calgary working in graphic arts, I started my first job in portraiture and soon after, started photographing nature and wildlife. This career has absorbed me for over 30 years because of my passion for the art of photography. The camera Duffy uses depends on Q: What skills do photographers need? whether he is shooting portrait or A: People skills as well as technical skills are important for a portrait nature photography. His cameras photographer. Nature photography involves a combination of can be fitted with different lenses. being in the right place at the right time and an eye for light Each photographic lens uses a combination of convex and and composition. concave lenses. Q: What was your most challenging photo? A: Taking pictures of intricate frosted and bevelled glass windows (windows with sloping edges) and doors for a product catalogue. Getting the subtle detail from the bevel and the frosted areas was definitely one of my most fulfilling accomplishments.
Q: What is your favourite part of being a photographer? A: I like the mixture of working with people and working on Snowy owl in flight near Tuxford, the computer. I also enjoy travelling to research nature and Saskatchewan wildlife pictures.
Q: What is the most challenging part of your job? A: Capturing the personality of my clients through a photo is the most challenging part of my job.
Storm Chasing near Old Wives Lake in the Chaplin Marshes, Saskatchewan. The storm shot was a Picture of the Month on the National Geographic website and was published in the magazine.
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BBIGIG IDEAI DEAS The visible light spectrum is made up of different colours. Colours hold special 5.0 meaning for First Nations and Métis peoples.
We live in a world filled with colour. From sunsets to clothing to the images seen in movies and television, colour defines and accents what people see. But what is colour? Why are there different colours? Why do certain lights change the colour of an object? Why are colour television images different from images in photographs and on film? So far, you have learned how light travels, reflects, and refracts. You have been considering the brightness of light and the images it forms. But these properties are only part of the story of light. You need to explore this world in colour. How many colours can you see in this sunset?
The human eye can see millions of colours. 5.1 The World in Colour
Make a list of all of the words you could use to describe a colour to someone else. Compare your list with others, adding any new words. Which words did most people include in their lists? Circle any words that have to do with light. How do you know that the words have anything to do with light?
Light and Colour Think of what it is like to wake up just before sunrise. There is enough light to see what is around you, but everything appears either grey or black. Then, when the sun rises, or you turn on a light, the room is filled with colour. So, light is necessary for your eyes to see colour, but why?
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Think about a light source such as the Sun. Your eye can detect light either directly from a light source or when this light source is reflected back to your eye by some object. What colour is a beam of sunlight? You probably answered, “White.” However, that’s not quite the whole picture. In the next activity, you will use refraction to find out for yourself.
P ROBLEM S OLVER
The Colour of Light
• Shine a light ray through a prism. What does your group observe? • Place a second prism in the path of light leaving the first prism. What does your group observe? • Turn the second prism in different ways. Compare the results. • Draw a coloured sketch in your Science Journal.
A prism is a transparent body often made 1 What colours did your group find in of glass, with ends that are equal and the light ray? parallel triangles, and other faces that 2 With your group, create a mnemonic are parallelograms. Recall what you that will help you remember the order have learned about the refraction of light in which the colours appear. through glass: Light changes direction when it enters or leaves glass. 3 What happened when your group In a small group, collaborate on the used two prisms in combination? following procedures and questions.
infoBIT Sundogs and Haloes CAUTION! A sundog, or solar halo, is a glowing, coloured image To best see a sometimes seen near the Sun. Small ice crystals that sundog, you should wear are present in the high atmosphere act like prisms and protective refract light from the Sun. This creates two patches of glasses (not colour on either side of the Sun. Sundogs are so named sunglasses!) probably because the Sun and the patches move to view the together in much the same way as a person walking Sun directly. a dog. Haloes can also be seen around the Moon.
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The Colour Spectrum
When you used a prism to refract a light ray, you were able to split light into its colours. A rainbow is created in the same way. A rainbow is formed as sunlight changes direction as it enters single raindrops. Inside the raindrop, the coloured rays are bounced off the inside wall, which acts like a mirror and reflects the rays back out again. As the rays leave the raindrop and return to the air, they change direction again. The result is a band of colours across the sky. The colours of light together form the visible light spectrum red, orange, yellow, green, blue, violet (ROYGBV). Every colour you see is a mixture of these colours. How can the spectrum produce hundreds of thousands of different colours? In the next activity, you will investigate this for yourself.
prism angle of refracted ray refraction normal normal angle of incidence emergent rays
incident ray Follow the path taken by light as it moves through this prism. Why were light source normals included in this diagram? Compare this with what happens when you shine a light along the visible light spectrum normal into a glass block and at other angles of incidence.
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I NVESTIGATOR
Mixing Coloured Lights
Before You Start ... Have you ever mixed coloured paints together to make different colours? For example, when you mix red paint and yellow paint together, you make orange paint. What do you think would happen if you mixed the same coloured lights? Would you get the same results? In the following experiment, you and your group members will work together to set up the equipment, Ready to investigate colour collaborate on the questions in the Analyzing and Interpreting section, 2 Work with two partners. If you are and reach a conclusion. using ray boxes, slide the coloured filter ahead of the one-slit baffle. If The Question you are using flashlights, use coloured What colours can you make using red, tissue paper or coloured plastic film green, and blue light? to make filters for each. Experiment with your apparatus until you get the brightest possible light on the white Materials & Equipment paper screen. • 3 light sources of equal intensity (flashlights or ray boxes) • 3 coloured filters of equal thickness (red, green, and blue) • white paper to act as a screen • red, green, and blue coloured pencils
Procedure
1 Predict what colours you would make by combining: a) red and green lights b) blue and green lights Step 2 c) red and blue lights 3 Combine all three coloured lights (red, Record your predictions in your green, blue) and shine them on the Science Journal. screen so that their beams overlap. What do you observe?
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4 Try different combinations of lights. 7 What did you need to do in order to Record your observations in a table. make your light more visible?
8 What, if anything, surprised you Keeping Records about some of the results? 5 Use coloured pencils to record your observations. Forming Conclusions
9 Make a diagram showing what each Analyzing and Interpreting combination of colours produces. 6 How accurate were your predictions How does your diagram explain about how coloured lights would why sunlight appears white? combine?
1 What colours are produced when white light is shone through a prism?
2 What do you think would happen if light of one colour, such as red, was shone through a prism? Explain your answer.
3 Imagine you have a choice of using any combination of red, green, or blue spotlights to decorate the front of your home for the holidays. Predict what would happen if you shone each of these combinations against a white wall. a) red and green b) red and blue c) green and blue d) all three colours combined
4 Explain why you can see a rainbow. Do you think it is possible to reach the end of a rainbow? Why or why not?
5 Using the list of descriptive words you made at the beginning of this section (see page 137), write a note describing your favourite colour. In your note, talk about how this colour makes you feel. Where would you use this colour: for clothing, in your room, in some other way? Why?
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AskAsk aa KnowledgeKnowledge AdvisorAdvisor Mary Lee: Colours of Life
For centuries people of the Plains have used colour for a variety of purposes in their traditional lifestyles. The Plains Cree are no exception. Mary Lee from Chitek Lake First Nation understands the symbolic and spiritual importance of colour for her people and uses colour for her beadwork. “When I was a girl, I used to trace my mother’s patterns [for my beadwork] but she told me to go outside. ‘Your teacher is out there.’ My mother meant that Mother Earth would teach me patterns and design,” said Mary, who proudly displays her intricate work. “My flowers have five petals. Why five? All the flowers that I could see had five petals. In the centre of my petals Traditional beadwork I use white to represent life. The petals are [beaded] in pink to represent life and beauty. The green in the leaves represents Mother Earth. The flowers are connected by strands of white beads to show that life continues.”
Mary prefers to be known as a knowledge advisor, although she is called an Elder by many people in her Cree community and other Aboriginal communities of Saskatchewan. A knowledge advisor is one who helps people to learn, to honour, and to live the old ways. The beauty of Saskatchewan’s flowers informs the design of knowledge advisor Mary Colour not only permeates Mary Lee’s work, but it is also strongly Lee’s beadwork. In a similar connected with her traditional life. She describes the significance way, the beauty that some of colour to her people. For the Cree, colours may have many scientists see in mathematics deep symbolic meanings and are often associated with the medicine informs their work in science. wheel. For example, colour represents the stages of life, the four directions, the four “races” of humanity, and the four aspects of the physical body. White in the medicine wheel is symbolic of old age, winter, the l a m n north, and the physical body. Beside white, the colour yellow is for e
o n
i
t t a
infancy, spring, the east, and the mind. In the quadrant next to yellow, o l m
red represents youth, summer, the south, and the spirit. Lastly, blue e sp l symbolizes adulthood, fall, the west, and the emotions. Mary iritua teaches children about colours to inspire them to see and understand the seasons. The Cree medicine wheel “We use colour in the medicine wheel to honour all created people,” uses blue and the quadrants says Mary. “That was why that teaching was given to the ancestors. form an “x.” There are The four races were created to teach us how to live in harmony.” many variations among the medicine wheels important to different First Nations peoples.
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5.2 Seeing Colour
The technological breakthrough that led to colour television involved making a screen that contained tiny dots of colour. When these dots glow in different combinations, different colours appear on the screen.
Adding Colours of Light Together Examine this close-up view of a Mixing coloured lights together to produce other colours can be television picture. You will see that explained using the addition model of colour. When red, green, the picture is made up of tiny and blue lights are put together in various combinations, as you dots of colour. saw in the Investigator activity, your eyes add these colours together and see an average, or secondary, colour.
blue infoBIT After-image Sometimes, after you have stared at a particular colour yellow cyan for a long time, you will see an “after-image.” Look at this circle for about 30 s, then, green red immediately look at a piece of magenta white paper. What do you see?
Primary colours Secondary colours
The primary colours of light are red, green, and blue. When you put all the primary colours of light together, you produce white light. Secondary colours of light are produced when you mix pairs of primary colours of light together. The secondary colours of light are yellow, cyan, and magenta. Yellow is produced by mixing green and red light. How are cyan and After 30 s, the cones in your magenta produced? retina that are sensitive to Recall that the retina is the lining at the back of your eye that red light had become tired. So reacts to light. The retina itself is made up of specialized cells. when you looked at the white Some of these cells are called cones. There are three types of paper, which reflects light that cones, each sensitive to different ranges of colour: red, green, and contains all the colours, you blue. When light hits the cones, the cones send messages to your saw light without any red. brain. The colour that you see depends on the type and number • How does this explain the of cones responding to the light entering your eye. colour you did see?
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Some people do not see all colours. The cone cells within the eye may be defective causing a condition known as colour blindness. Look at the images on the left to determine if you are colour blind.
1 If you stare at one colour for a while, then look at a white People who are green and yellow page, you will see a different coloured after-image. Use the colour-blind will not see the diagram of colour addition on the previous page to predict number “25.” which colours you would see after staring at each of the following colours for a while. Explain what is happening within your eye in each case. a) blue b) green c) red d) yellow (Hint: What colours make up yellow?)
People who are red and green colour-blind will not see the number “5.” 5.3 What Colour Is It?
Have you ever tried on different pairs of sunglasses? Depending on the colour of their lenses, sunglasses can give the world a coloured tint. Some can make everything look yellowish. Others seem to make greens and blues more intense. Why does this happen? Is this an effect on your eyes or on the light? In your Science Journal, write down what you think might be happening as light passes through a coloured lens or filter.
Subtracting Colour A filter lets some parts through and keeps other parts out. Sunglasses are not only “cool.” They A coffee filter lets water and the flavour of coffee pass into a pot, subtract some part of the light. while keeping out the coffee grains. Coloured sunglasses are like filters that act to take out, or subtract, some part of light. You know that white light contains all of the colours in the spectrum. But why do objects have colour? For example, why is a tomato red or why is the grass green? What would you see if you looked at a red tomato using a filter that allowed only green light to pass through? if you looked at green grass through a filter that allowed only blue light to pass through? Start exploring!
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I NVESTIGATOR
Looking Through Coloured Filters
The Question 6 Repeat steps 4 and 5 with the What effect do coloured filters have on remaining filters. coloured objects? 7 Focus a narrow light ray onto the prism to produce a spectrum of colours on the white screen. Materials & Equipment 8 Use the coloured filters one at a time • ray box with a single-slit opening to direct a light ray onto the prism. • coloured filters: red, green, and blue Record what you see. • white paper to act as a screen Keeping Records • coloured pencils (optional) • coloured paper: red, green, and blue 9 Make a table of your observations.
• prism Analyzing and Interpreting
10 When white light passes through a Procedure coloured filter, what colour do you
1 Set up your ray box so the beam of see? What happens to the other light falls on the white screen. colours of light?
2 Darken the room, and examine the 11 When you looked at the red paper coloured papers in the white light one through the green filter, what colour at a time. Record the colours you see. did you see? Why do you think this happens? 3 Use the coloured filters one at a 12 What effect did each coloured filter time to focus have on the light passing through the the light ray prism? How do you know? onto the white 13 Describe or draw what you would see screen. Carefully if you looked at a traffic light through observe the colour each of the following coloured filters: that falls on the screen. red, green, and yellow.
4 Select a coloured filter. Predict the 14 What applications use filters to absorb effect that the filter will have on each unwanted light? piece of coloured paper.
5 Look at each piece of coloured paper Forming Conclusions
through this filter. Record your 15 How do coloured filters work? Write a observations. conclusion that answers the question.
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reSEARCH The Subtraction Model of Colour You have seen what happens when you added different light Graphic artists deal with images colours together by projecting coloured light rays on top of for the computer screen such each other. When you subtracted light colours by passing light as website layouts and Internet through filters that block, or subtract, certain colours, something advertisements. Graphic artists quite different was taking place. What determines the colour of also deal with images for printing something when colour subtraction is used? such as posters, billboards, and Recall that the primary colours of light are red, blue, and business cards. Research which green. If an object appears red, then its surface is reflecting red colour model graphic artists to your eye, and absorbing (subtracting) the other two primary use for websites and which colours, blue and green. If an object appears blue, which colours model they use for printing. have been subtracted? • What is the difference between 16, 24, and red green blue red green blue 32-bit display settings red for viewing colour? green • What is the optimal display setting? Photo courtesy WorldSkills International Photo courtesy WorldSkills Graphic designers work with images to be displayed on the Internet and for printing. The subtraction model of colour helps explain why objects are coloured. Pigments in objects absorb (subtract) certain colours and reflect others. You see a tomato as red because the chemicals in the tomato’s skin absorb all the colours except for red. Only the red light is reflected to your eyes. Green grass absorbs all light except green, which is reflected. Special lights for growing plants look reddish. Why wouldn’t they be coloured green?
The primary colours in pigment, however, are different. They are magenta, cyan, and yellow. Look at the illustrations of the colour subtraction model on the next page. What do you notice about how the primary colours for light and pigment are related? Can you explain this relationship? In fact, colour photographs in print media (for example, books and magazines) actually use just four colours: magenta, cyan, yellow, and black. Can you guess why black is used? When white light shines on these colours, the pigments work in the same way a coloured filter works. Each primary colour of pigment absorbs, or subtracts, one of the primary colours of light and reflects the other two. This explanation of the process is called the subtraction model of colour.
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Colour subtraction applies to the secondary colours as well. Remember that secondary colours are those produced by mixing any two of the primary colours together. For example, blue yellow light is a mixture of red and green light. magenta cyan A yellow object reflects a mixture of both pigment pigment colours and absorbs, or subtracts, the colour blue, so your eye sees yellow. There are many applications of coloured red green lenses. Glasses have many different lens colours such as yellow, brown, and copper. Yellow lenses are good for night driving or for seeing when skies are overcast. Brown and yellow pigment copper lenses are preferred for activities such as driving or skiing because they reduce bright light and sharpen images. Sunglasses not only enhance vision, they also filter the harmful Notice what is happening when the secondary colours are mixed ultraviolet rays from the Sun. together. Magenta pigments absorb green light and reflect a mix of red and blue light; cyan pigments absorb red light and reflect a mix of green and blue light; yellow pigments absorb blue light and reflect a mix of red and green light. 1 What are the primary colours of light? What are the secondary colours of light?
2 a) How does the eye detect colour? Explain, using the addition model of colour. b) How does a painting show a colour image? Explain, using the subtraction mode of colour.
3 Imagine you are an actor in a horror film. Your costume calls for scary, red contact lenses to change the colour of your eyes. When you examine the lenses, you see that they are clear in the centre, with only a ring of red colour. Why do you think this is so? What would you see if the contacts were completely red? completely yellow? Explain, using the subtraction model of colour.
4 In photosynthesis, plant cells convert light energy into chemical energy they can store as food. If a plant has green leaves, which colour(s) of light is (are) being absorbed by the plant? What does this suggest about the type of artificial lighting that should be used to grow plants indoors?
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AskAsk anan ExpertExpert Jeff and Kathleen Coleclough: Preserving Traditional Ways
The traditional ways of Ojibwa people live on in the work of Jeff and Kathleen Coleclough. The couple are Métis people of Ojibwa, Cree, Assiniboine, and European, French Canadian, and African descent. From their home in Riceton, Saskatchewan, they own and operate “Kakwa,” an initiative that involves the production of replicas, art, regalia, and artifacts for education and preservation of First Nations and Métis traditional skills. In addition to these cultural art forms, Kathleen has also learned to make traditional dyes. She learned the art from a Blackfoot Elder. Traditional dyes are colouring solutions made from natural plant materials. The dye uses the pigments from plant parts such as berries, bark, or root, boiled and mixed with a mordant. A mordant, or fixing agent, is used to set the dye so that the colour does not fade. Jeff Coleclough is tanning Kakwa is the Cree word for “porcupine.” First Nations a hide. peoples of the plains used dyes to colour the quills of porcupines. These quills were then sewn into clothing or used to decorate willow baskets. Following traditional ways, Kathleen always offers Tobacco before she picks the plant parts needed to make her dyes. This action expresses respect for her relationship with Mother Earth.
Colour Plant Part Used Mordant red buffalo berry large taproot of buffalo berry plant yellow cottonwood buds honey wild sunflower cattail root black black walnut clay maple tree inner bark purple blackberry root of the blackberry plant saskatoon berry root of the saskatoon berry plant blue (not a beech tree hardwood ashes true blue) Porcupine quills are delicately tinted using natural dyes.
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5.4 Check Your Progress reSEARCH One of the most important 1 Make a table to show the primary colours of light and how applications of the subtraction they combine to form the secondary colours of light. model of colour is spectroscopy. A device called a spectrometer 2 With white light, shadows appear black. Describe how can be used to measure which shadows would appear if an object were placed between a colours of light are absorbed combination of red and green light, or a combination of (subtracted) by a substance. blue and green light. Each substance has its own 3 Copy these sentences into your Science Journal. pattern of light absorption, Complete them using the terms “reflection” or “refraction.” which can be used as its a) You can see yourself in a mirror because of the “fingerprint.” of light. • Investigate how spectroscopy b) A prism can be used to split light into its colours works. because it causes of the light. c) When an object in water appears to bend, this is due to the of light as it passes through a different medium. d) When an object appears red, this is due to the of the red light from the object.
4 Work with students in an art class in your school to Using a spectrometer investigate how artists use the subtraction model of colour to inform their work in dance, drama, music, or visual arts. • What are some of the ways it is used in science and industry? 5 Based on what you have learned about colour, can you infer why the sky is blue and a sunset is red? You may have to do research to answer this question.
6 Examine the parts of a strawberry plant Plant Colours shown on this page. a) Copy and complete the table to the right. Part of the plant Colour (Explain, using the colour subtraction model.) b) What is the benefit of the colour of each part to the plant? For each part, make a leaf hypothesis to explain why it is coloured flower as it is. Plan an experiment that would fruit let you test each hypothesis.
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BBIGIG IDEAI DEAS Visible light is only one part 6.0 of the electromagnetic spectrum.
Technology that uses the electromagnetic spectrum
What do microwave ovens have in common with radios? How about X-ray machines and cellular telephones? All of this technology makes use of radiation the human eye cannot see, sometimes called the invisible spectrum. The term used to refer to all forms of radiant energy is electromagnetic radiation. The electromagnetic spectrum includes visible light, as well as other waves, such as infrared radiation, ultraviolet radiation, radio waves, and X rays. In order to learn more about these other forms of radiation, it helps to take a step back and consider more aspects of light itself.
6.1 The Wave Model of Light
An important part of science is developing models. Models are based on what can be observed about the characteristics and properties of something. They help make it easier to understand complex concepts. What kind of model could be used to help describe light? A model commonly used is the wave model of light.
Developing the Model On the following pages is an outline of the information and the properties of light on which the wave model of light is based. Like any scientific model, it provides explanations for many aspects of observed phenomena.
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Properties of Light infoBIT The light that you live with and enjoy on a sunny day can be Laser Light looked at in terms of the scientific properties discussed so far in this unit. These properties are reviewed in the drawing below. Lasers produce a fine beam of light that travels long distances Light consists without spreading out. Laser of colours that can be Light is a form of energy. Light moves outward light is made of waves that have split apart and brought in every direction from back together. exactly the same wavelength, a light source. Light is something and each wave is in step with the our eyes can detect. next. The beam concentrates light energy into a small spot Light moves in straight lines. so that lasers can be used to cut metals and perform delicate Illuminance Light can be eye surgery, as well as provide decreases with distance reflected and refracted. an input module for many from its source. electronic devices. (The term “laser” is an acronym for Light The properties of light Amplification by the Stimulated Emission of Radiation.) Arthur The Properties of a Wave Schawlow (1921–99), who In the wave model of light, light is assumed to show the was educated in Toronto, properties of a wave. These properties are reviewed in the received the Nobel Prize in diagram below. Physics as its co-inventor.
• A wave is a form of energy. wavelength—the distance between the top of one wave and the top of • A wave moves out in every direction. the next rest position
amplitude—the height of the wave from its middle rest position to its highest point frequency—the number of times Working with a laser and fibre optics a wave source or medium vibrates in a given unit of time
The distance between the top of one wave and the top of the next is called the wavelength. All waves have a wavelength. When the wavelength of light is measured, it turns out that each colour has a slightly different wavelength.
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Exploring Properties of a Wave
The Slinky®, popular with young children, • Create another wave of double the is a good way to see the properties of waves frequency of the low-frequency wave. in action. How can you use a Slinky® to Draw a diagram of the wave titled demonstrate the properties of waves? “High-Frequency Wave” and label the three parts of the wave. • In the hallway or in an open area in the classroom, stretch your Slinky® to • Keep the frequency of the wave the about twice the original length—do not same as the low-frequency wave, kink, knot up the Slinky®, or stretch but double the amplitude. Draw a it beyond its elasticity. diagram of the wave titled “Double Amplitude Wave” and label the • One student holds one end of the three parts of a wave. Slinky® still and the other student moves the other end slowly from side 1 Which takes more energy to produce: to side. Create a low-frequency wave. the low-frequency wave or the high- Draw a diagram of the wave titled frequency wave? “Low-Frequency Wave” and label the three parts of the wave. 2 What happens to the size of the wavelength when frequency is increased?
Wavelength and the Electromagnetic Spectrum The visible spectrum extends from red light at one end to violet light at the other. The wavelength is not the same for different colours of the spectrum. It ranges from 700 nm (nanometre, 1 nm = 0.000 000 001 m) for red light to 400 nm for violet light. You can see that some of the light waves are short and some are longer.
The visible spectrum is a part of the electromagnetic spectrum.
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What You Cannot See Does the spectrum end with what our eyes can see? You have heard the terms ultraviolet or infrared. What else might be in the electromagnetic spectrum that you cannot see? In the next section, you will explore the other components of the electromagnetic spectrum and their importance.
1 How might your life be different if light consisted of only one wavelength? Pick a wavelength from the visible light spectrum and reflect on how your natural surroundings would be affected. A
2 What properties of light are similar to the properties of the waves in A and in B? How do you know? B
3 Draw a diagram of a wave on a piece of graph or grid paper. Your wave should have an amplitude of 4 cm and a wavelength of 10 cm.
6.2 The Invisible Spectrum
If you were to touch the back of each of these What changed In a moment, this person will roll over in his sleep so students, which coat would feel the warmest? the colour of this all of his body will be back in the shade. How did he Can you explain your choice? person’s skin? detect the sunlight on his skin while he was sleeping?
Think about the situations shown here. Did they involve seeing colour or forming an image? In your Science Journal, write your own explanations for these events. Add any other observations you may have made about the effect of the invisible components of the electromagnetic spectrum.
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reSEARCH Electromagnetic Radiation Polarized lenses benefit The electromagnetic radiation spectrum includes visible light, human vision. What does the as well as other waves, such as infrared radiation, ultraviolet polarization of lenses involve? radiation, radio waves, and X rays. The wave model of light What are the benefits and is used to explain all electromagnetic radiation. The movement limitations of this technology of waves in straight lines is called rectilinear propagation. Just as in the following areas? each colour has its own wavelength, so does each of the invisible components of the electromagnetic spectrum. However, you • water sports see only visible light. Your eyes are sensitive to the wavelengths • skiing from red to violet. You cannot see the other waves because • driving your eyes are not sensitive to their wavelengths.
Uses of the Electromagnetic Radiation Spectrum
radio waves microwaves
Radio waves are used in Microwaves have a shorter wavelength Infrared waves are longer than communications around the world. than radio waves. They also carry more visible light and so have less energy. International agreements assign certain energy. When focussed on food, they You can detect infrared waves as frequencies to different uses, including cause the particles inside the food to heat on your skin. AM/FM radio, television, cellular phones, vibrate. This vibration produces heat, short-wave radio, and radar. so the food cooks from the inside out.
South-facing windows and skylights allow infrared radiation to help heat homes. With AM radio waves, the amplitude of the signal is varied. With FM radio waves, the frequency of the signal is varied.
Devices that detect infrared radiation are used to locate sources of heat.
Electromagnetic radiation strikes Earth in many different forms: radio waves, microwaves, infrared rays, visible light, ultraviolet light, X rays, and gamma rays.
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Useful Radiation Technologies Although the human eye cannot detect the invisible parts of the electromagnetic spectrum, we have found ways to use the energy they contain. As you examine the uses of the various parts of the spectrum, note which parts have more energy. As a clue, remember that the shorter the wavelength, the higher the frequency of the wave. High-frequency waves carry more energy. Which of the technologies shown here have you or a member of your family used?
visible light X rays infrared waves ultraviolet gamma rays
Ultraviolet (UV) waves have short X rays and Gamma rays have very X rays can be used to make images of the wavelengths with more energy than short wavelengths and carry a large interior of the body, because they will be visible light. Most ultraviolet radiation amount of energy. Even low-energy X rays slowed down by very dense tissues such is absorbed by Earth’s ozone layer. can penetrate soft tissues, although they as bone. Gamma rays and high-energy cannot pass easily through bone. High- X rays are used to destroy cancer cells. energy X rays and gamma rays can pass easily through soft tissues and bones, but cannot penetrate lead 5 cm thick.
Ultraviolet waves can penetrate and harm the skin, causing tanning and burning, and increasing the risk of skin cancer. Sunblock creams are opaque and block ultraviolet rays. Gamma ray trails in a cloud chamber.
Sunglasses with UV filtering protect the eyes from damage. There is evidence that daily exposure to UV radiation increases Fractures and other medical conditions the risk of developing cataracts. can be shown by X-ray technology.
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Careers and Profiles Dr. Lisa Van Loon: Industrial Scientist
Lisa Van Loon realized at an early age that she wanted to become a scientist. After completing high school in Saskatoon, Saskatchewan, Lisa attended the University of Ottawa (Canada’s bilingual university) where she was intrigued by the ability to control laboratory experiments. Lisa’s experience in taking first-year chemistry inspired her to continue her studies in the field of chemistry, in both English and French. Upon completion of a B.Sc. in chemistry, Lisa continued in the graduate program at the University of Ottawa doing research in the field of Environmental and Chemical Toxicology. Lisa completed her M.Sc. and continued her studies at Ohio State University, where her Ph.D. research project in atmospheric chemistry investigated the role of aerosols in climate change. After post-doctoral research work at the University of Georgia, Dr. Van Loon returned home to Saskatoon to work at the Canadian Light Source (CLS). The CLS is a synchrotron and is featured on pages 168–169 of this unit. As a physical and analytical chemist, Dr. Van Loon applies her knowledge and experience designing experiments with the synchrotron to solve industrial environmental problems. She primarily works with mining companies in Saskatchewan Canadian Light Source dealing with environmental science. Efficient waste management is a priority for mining companies and the synchrotron has helped identify the state of metals in mine tailings. Another application of the synchrotron to the mining sector is the characterization of hazardous dusts and aerosols related to worker occupational Health and Safety issues, an area close to Dr. Van Loon’s personal research interests in atmospheric aerosol chemistry. As a scientist at the CLS, Dr. Van Loon has many opportunites to collaborate with other scientists and people from around the world. When Lisa is not at work, she enjoys her pet Mine tailings, such as those in this picture, can be analyzed dogs. She also plays several musical instruments using the Canadian Light Source to find out what metals and has started learning the violin. are present.
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Wireless Technologies
Wireless technologies transfer information • In small groups, summarize the various through the air. Distances may be short or types of wireless technologies in your very long. Many people rely on wireless homes and the quantity of devices. technologies throughout their homes to Then, compile a class survey. perform different tasks. You may use the • From the class survey of wireless following in your home: WiFi network, technologies in the home, construct cellphones, pagers, PDAs, GPS, a bar graph of the quantity (vertical telephones, satellite dish, TV remote axis) and type of wireless technology control, garage door opener, wireless (horizontal axis). mouse, radios, CB radios, ham radios, pagers, wireless gaming controllers and portable gaming devices, Bluetooth 1 What impact does wireless technology devices, wireless speakers or headphones, have on you and your family? laptops, and wireless weather sensors. 2 Do any of the wireless devices interfere with each other?
3 What other items would you prefer to be wireless?
4 Research the environmental impacts of wireless technologies. Possible questions to examine include: a) How might the use of wireless technologies affect human health? b) What is the life cycle of wireless devices, and are they recyclable or do they end up in landfill sites? c) How does wireless technology support social change and help Wireless technologies exist all around us! monitor the environment?
• Survey the availability of wireless technology to you, your family, and What Do They Mean? your class members. WiFi wireless fidelity PDA personal digital assistant • Record the quantity of individual items: GPS global positioning system for example, four wireless phones. TV television CB citizen’s band • If possible, record the frequency at which each device operates, for example, 2.4 GHz phone.
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I NVESTIGATOR
Combating Electromagnetic Radiation— Effectiveness of Sunscreen
The Question Analyzing and Interpreting
How does suncreen work? How important 5 Which colour bead/disk responds is the use of sunscreen to health? quickest to the UV light?
6 Are some colour changes easier to Materials & Equipment see than others? • UV beads or disks (4 different colours and 12 of the same colour) Part 2: Process • access to sunlight or to UV light 7 Draw four circles on a piece of paper. • at least two types of sunscreen including 8 Label one circle “control.” Label the SPF 15, SPF 30, and SPF 45 other circles SPF 15, SPF 30, and SPF 45. • timer • Q-tips™ (at least 3) 9 Use a Q-tip™ to apply SPF 15 to the surface of a bead/disk. Place the bead/ disk into the circle labelled SPF15. Part 1: Procedure 10 Using a clean Q-tip™ each time, apply 1 Lay the beads or disks on a piece of the other strengths of sunscreen to white paper. (Use at least four colours.) beads/disks of the same colour. Place the disks into their respective circles. 2 Expose the beads or disks to UV light,
and record your observations in a 11 Expose all four beads/disks for 3 min table similar to the one below. to sunlight or to a source of UV light if available. Use a scale of 1–5 to rate 3 Remove the UV light and record your observations. Record the time (in the variation in colour, where 1 is the seconds) it takes for the colour to fade. lightest and 5 is the darkest colour.
4 Repeat steps 2 and 3 and record your CAUTION! results for Trial A and Trial B. Always use protective goggles when working with UV light. UV Colour Colour Colour Colour Colour Bead/Disk 1: 2: 3: 4: colour trials A B A B A B A B colour the bead/disk becomes time (s)
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12 Record your results in a table similar Analyzing and Interpreting to the one below. 14 What was the average of the numbers Trial Control SPF 15 SPF 30 SPF 45 recorded for SPF 15, SPF 30, and Bead SPF 45? 1 15 Draw a bar graph of the Colour vs. SPF. 2 16 Suggest ways that the colours shown 3 with the different sunscreen strengths 4 could be related to sunscreen use Average by humans.
13 Repeat this process three times. Each Forming Conclusions time reapply the same SPF sunscreen 17 Based on your findings, which SPF values and keep the same control blocks the most UV light? What would bead/disk. be the result of a person using only a low SPF sunscreen being exposed to sunlight for a long time?
1 Compare X rays to radio waves and visible light waves to infoBIT radio waves. How are they different? Night-Vision Technology 2 How do sunscreen Night vision is the ability to see lotions protect your in a dark environment. Human skin from the Sun’s vision is confined to a small rays? Which part of portion of the electromagnetic the electromagnetic spectrum (the visible spectrum). spectrum can cause Therefore, optical devices used skin damage? at night must either intensify the small amount of light available or include some of 3 Debate this question with your classmates: Do other forms the non-visible electromagnetic of electromagnetic radiation, such as microwaves or spectrum of light. Some night- infrared waves, have the property of reflection? What vision technologies use: support do you have for your opinion? • infrared radiation 4 Why do you think X rays and gamma rays are used to • ultraviolet radiation treat cancer?
5 Briefly describe three positive effects and three negative effects of wireless technologies.
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6.3 Sources of Light and Other Forms of Electromagnetic Radiation
Think of yourself on a sunny day sitting in a classroom. What sources of light are there? Can you think of other sources of light? Look at the photos below and classify each source of light as either natural or artificial. Share your classification system with others in your class. Is your definition of “natural light” the same as that of your classmates?
B C
A D E
Natural and Artificial Sources of Electromagnetic Radiation Do you realize that every day you are bombarded by different forms of electromagnetic radiation from natural and artificial sources? Can you identify all the different sources of electromagnetic radiation that you experience in one day?
Natural Sources During the day, our primary natural source of light is the Sun. Think about being outside in the sunlight. What do you notice as the Sun shines on your skin? What effects does exposure to the Sun have on your skin? What about the time you hiked a long distance on a hot, cloudy day and were surprised to discover later that you had a sunburn? How could that happen on a cloudy day? Most forms of electromagnetic radiation are produced in space by the Sun and other stars.
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Bioluminescence The phenomenon of bioluminescence happens when a naturally occurring chemical reaction (see chemiluminescence below) takes place within a living organism and results in the emission of light. Fireflies and glow-worms (their larvae) use a complex reaction to make their tails glow. Other organisms such as shrimp, squid, and starfish use bioluminescence for communication. Some forms of fungi that cause wood decay glow at night. This creates the eerie nighttime phenomenon known as foxfire. Bioluminescence is a naturally occurring source of light.
Some marine organisms produce light through chemical reactions in cells.
Artificial Sources All types of electromagnetic radiation can be artificially produced. You are most familiar with devices used to produce visible light. Imagine the moment when your ancestors first used firelight to push away the darkness. Think about all that you do at night or indoors that you could not do without light.
Chemiluminescence Items that glow in the dark (produce light) due to a chemical reaction are described as chemiluminescent. When a glow stick is bent, the two liquids inside the stick mix, resulting in a chemical reaction. This chemical reaction causes the stick to glow for several hours until the chemical energy is depleted. In crime scene The mixture of two liquids inside investigations, blood is detected from a luminol test. Luminol the stick causes a reaction that glows blue when it chemically reacts with iron in the blood. produces light.
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Phosphorescence Have you ever wondered why some stickers or plastic shapes glow in the dark? These items when exposed to light absorb it and then glow. Phosphorescent materials absorb light energy, store it for long periods of time, and then release it as a form of light. Other items that use phosphorescence are photographic-darkroom timers and wristwatch dials. Fluorescent sources and phosphorescent sources are similar; they both absorb light, store it, and release it. The difference between the two is that fluorescent sources immediately release their light energy while phosphorescent sources take longer to emit the light. Phosphorescent materials allow the dial to be read. Electroluminescence When electric current passes through a material such as a semiconductor and light is emitted, the phenomenon is known as electroluminescence. Light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) are technologies that are applications of electroluminescence (LEDs are covered in more detail on the next page). Many wristwatches use electroluminescence to turn on a backlight. This produces a blue or green glow, allowing the numbers to be seen in the night.
OLED vs. LCD? Liquid crystal displays (LCDs) are used every day to display information electronically on computer monitors, television screens, and instrument panels. However, the OLED may become the next trend for consumer TVs. An OLED is a source of electroluminescence in which the light-emitting layer is an organic compound. Many other applications of the OLED technology exist—mobile telephones, laptop and stereo displays, car navigation systems, and billboards. Some of the advantages of this technology over traditional LCD screens are: • simpler and cheaper manufacturing process • thinner screens (less than 1 mm thick) • high contrast and brightness (can see the screen with direct sunlight on it) • high response speeds and wide viewing angles
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Common Lighting Devices infoBIT What types of devices are used today to make light? If you look Lighting Up the School around your classroom, you will probably see either incandescent or fluorescent lights. An incandescent light bulb uses electrical Count the number of light energy to heat a thin wire thread, called a filament. Because bulbs in your classroom. the filament is too thin to carry the electricity easily, it • How many classrooms overheats and glows white-hot. You see this glow of the are in your school? white-hot filament as light. • Estimate the number of Compact fluorescent lamps contain an opaque tube that is bulbs in the building. coated on the inside with a fluorescent material. The tube is You can see how quickly the filled with argon gas and a small amount of mercury vapour. cost of supplying energy to all When you turn on the electricity, UV light is produced in the of these light bulbs would add tube. When the UV light hits the fluorescent material, the UV up. The easiest way to reduce waves are absorbed. The material then releases the absorbed this cost is to use bulbs that are energy as visible light. very efficient at transforming electrical energy into light energy, without wasting a lot of energy as heat.
An incandescent light bulb A compact fluorescent lamp
Another important new form of lighting is the light-emitting diode (LED). LEDs do not have a filament the way an incandescent bulb does. Electrons which are tiny negatively-charged particles, move through a semiconductor material and emit light. LEDs are commonly found in electronic devices. The LED is now being made into clusters of up to 180 individual LEDs to make a light bulb. The clusters of LEDs are encased in a diffuser lens, which disperses the light in wider beams.
Decisions About Technology If you look around your home, which type of lighting is being used in the different rooms? Why do you think this is so? People make decisions about the technology they buy and use for many LED lights in a cluster different reasons, including convenience, appearance, and cost. Cost involves not just the purchase price but also how much it costs to operate the technology. Energy can be a major cost in the manufacture and use of any technological system. In the next activity, you will compare the efficiency of an incandescent light bulb with that of a compact fluorescent bulb.
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D ECISION M AKER
What Type of Light Bulbs Should I Use?
The Issue 3 Be sure to compare your results with How can you demonstrate which of the those of others in your class. Should following is more efficient: an incandescent their results be similar to yours? bulb, a compact fluorescent light bulb, or 4 Through research, determine the pros an LED bulb? and cons for each type of bulb.
Background Information In Your Opinion 1 Not all the energy used in a light bulb 5 What do the results of your experiment produces light. Much of it ends up as tell you about the efficiency of these “waste heat.” You only have to put types of light sources? your hand near a glowing incandescent bulb to find that out. So, one way you 6 Based on your experimental findings could measure the efficiency of light and your research, what type of bulbs is to somehow determine how lighting would you recommend for much heat they produce. your school? for your home? for an office building? 2 Plan and carry out an experiment that compares the waste heat of an incandescent bulb, a compact fluorescent bulb, and an LED bulb. Here are some questions you might want to consider. ? a) What variables do you need to ? control in order to conduct a “fair test”? b) What materials and equipment will you need? c) Will you record the results of your ? data in a table? in a graph? d) Why should you repeat your experiment several times?
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try this at HOME The Better to See You
What type of lighting do you prefer: 2 Which light shows my best look? incandescent or fluorescent? Try these Put on your favourite outfit. Ask others experiments with your family or friends. You to compare the colours of your clothing will need both sources of light as well as some under each artificial light source to how sunlight! (Hint: How could a camera help you?) they appear outside in the sunlight.
3 Which light should I use? Get a strip of colour samples from a paint store. Compare how these colours appear in sunlight, in incandescent light, and in fluorescent light. Which light gives you the best idea of how the colour would look in your room? Choose a colour you might like for your room.
4 Based on your results, which type of light makes objects (and you!) look Procedure most pleasing?
1 Which light flatters me? On a scale of one to five, ask others to rank how your skin and hair look, in sunlight, in incandescent light, and in fluorescent light.
Feedback Stability • comment from • structure of swing person on swing Lighting Systems • safety of swing All lighting is designed as part of a system to perform a needed function. As in any system, Output • more energy there must be the following components: to swing • Input—Who decides what is needed? Who plans the system? What type of light source will be used? • Output—What is the desired result? How is the light to be useful? • Feedback—What checks and controls the system? Input • push from a friend • Stability—How reliable is the system? Does it do the job? What other systems can you describe that use this model of a system? What are the components in a lighting system? What do they do?
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P ROBLEM S OLVER
Analyzing a System
Lighting systems generally follow the • Read the article on stage lights below. model of a simple system. The input comes • Construct a diagram that identifies the from something or someone. The output input, output, feedback, and stability is the action or result of the input. The of this system. feedback is a control system that provides 1 Which part of this system is the most a path for data between output and input. complex? Why? Feedback depends on sensors to measure the output to keep it within set limits. 2 Which part of this system requires the The stability of a lighting system helps most care? Why? to maintain the system. A rock group is doing a series of tours. 3 Where does this activity fit into your The lighting technicians must constantly overall diagram of the system? set up, test, operate, then take apart the lighting system.
tage-lighting systems are at the heart of Lights! Stheatrical performances and rock concerts. They are often what make these events successful. Action! And as any student of biology knows, with every heart, there has to be a brain. When it comes to stage lighting, the “brain” is actually made up of many people. First, a lighting designer plans how to light the stage. To make the lighting effective, the designer must understand the behaviour and nature of light and colour. Many lighting designers use computers to help design their stage-lighting schemes. The lighting technician uses a console (a computerized board that holds the controls for all the lights) to control the lights. This is a challenging job. The lighting technician must be ready to deal with computers crashing, cables breaking, equipment wearing out, and working at great heights! A set may have over a hundred lights, many of them hung several metres above the stage. The lights are connected by cables of various sizes and are hooked up to switches (called dimmers) that control the brightness of the lights. Coloured filters set the right colour for the scenes. Besides the designer and the technician, there are a number of people who set up and adjust the lights.
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1 List four light sources and identify whether they are natural reSEARCH or artificial. Describe briefly how each source produces light. Ultraviolet light damages 2 Here are two approaches used by the makers of detergents. skin. So, why do people go to Explain how each approach uses light to make a white tanning salons and lie under T-shirt look whiter. ultraviolet light or go sunbathing a) Some detergents contain fluorescent materials. at the beach? Are there any b) Some detergents contain blue pigment (think back to positive aspects to the use the subtraction model of colour). of tanning beds?
3 Think about where you like to sit and read. What light • Conduct a survey to find out source are you using? What, if anything, would you how many students in the change about this light if you could? class know about the risks from ultraviolet light and how many consider a 6.4 Check Your Progress “good tan” to be worth it. • Do research to find out if
1 Explain similarities between the wave model and actual tans have always been observations of light-related phenomena. considered fashionable.
2 The electromagnetic spectrum ranges from low-energy to high-energy waves. Which of the following would you expect in a high-energy wave? Which would indicate a lower-energy wave? Provide examples to support your answers. a) a long wavelength b) a short wavelength
3 a) Where does infrared radiation appear on the electromagnetic spectrum? b) Can you detect infrared radiation with your body? If so, how? c) What is the importance of infrared radiation to you? d) Provide answers to parts a)–c) for ultraviolet radiation.
4 How is light being used in this doctor’s office? Name as many ways as you can. Add to your list any other ways you can think of in which light plays a role in medicine.
5 Which source would give the greatest amount of light: a 100-W incandescent bulb or a 23-W compact fluorescent bulb? Explain your answer.
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SCIENCE • WORLD Canadian Light Source
The Canadian Light Source (CLS) is a synchrotron located in Saskatoon, Saskatchewan. The synchrotron creates an intense beam of light that is a million times brighter than sunlight. The synchrotron is used for research to find solutions to challenges in health, the environment, and advanced materials science. A synchrotron produces extremely bright light by using radio frequency waves to move electrons (tiny negative particles) to nearly the speed of light. Powerful electromagnets alter the course of the electrons so they produce a brilliant light beam. The highly focussed light beam is very intense. If it were directed at a block of steel, this light beam would burn right through the steel! The light includes different parts of the electromagnetic radiation spectrum such as infrared, ultraviolet, or X rays. These different spectra are directed down beamlines by devices called monochromators (which act like prisms). Scientists choose their desired wavelength to study their samples in small laboratories located at the end of each beamline (see the figure below).
LINAC booster ring storage ring
beamline
end station
The Canadian Light Source: How a synchrotron works
The CLS synchrotron has four main parts. 1 The Electron Gun and Linear Accelerator (LINAC) — The process begins where high-voltage electricity passes through a heated cathode creating pulses of electrons. The linear accelerator boosts the electrons’ energy, moving them very close to the speed of light (3.0 ϫ 108 m/s). 2 The Booster Ring — The electrons travel around the 103-m ring approximately 2.5 million times in one second. The electrons have enough speed after passing throught the LINAC, but not enough energy. The booster ring boosts the energy by more than ten times.
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3 The Storage Ring — Electrons circulate in the storage ring for 4 to 12 h producing photons of light. Photon ports on the ring allow the light to travel down the beamlines. Superconducting radio frequency (RF) cavities replace energy lost by the electrons as they emit light. 4 The Beamlines and End Stations — Light travelling down the beamline first passes through a monochromator. The monochromator separates the wavelengths of light for the scientist, delivering a unique wavelength of light, such as infrared or X rays. Bird’s-eye view of booster and storage rings The selected wavelength is then focussed by lenses or mirrors onto the sample in the experimental endstation. Each endstation consists of a sample holder and detection system and a bank of computers through which the experimental data are recorded.
Saskatchewan Students Using the Synchrotron Each year, the CLS staff work with students and teachers from several schools throughout Saskatchewan conducting scientific experiments in a project called Students on the Beamline. Students from Lloydminster Comprehensive High School studied the taste of different honey types that had been characterized by using synchrotron soft X rays. Students from Avonlea School conducted X ray absorption spectroscopy experiments comparing the differences in trace elements among soil samples. Students from Centennial Collegiate in Saskatoon are using soft X rays to study the effects of acid rain on the chemical composition of soil.
In Your Opinion
1 Biodiesel is produced from oils such as canola, flax, and soybean, and even from animal fat. It is less toxic than table salt and biodegrades as fast as sugar. Infrared spectroscopy, like that at the synchotron, can be used to determine the oil content of the seeds. This information can lead to the development of seed varieties with a higher oil content. What problems does the manufacturing of biodiesel create? What problems would the production of biodiesel solve?
2 Research another scientific benefit from the synchrotron. Give a specific example of how this discovery has helped us.
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Project Solar-Powered Systems and Wavelength
Getting Started Before You Start ... Scientists and engineers measure electrical You are a member of the “solar-drag-car” and energy in kilowatt-hours (kW h). In one year, “SK-solar-boat” research teams for a major people on Earth use more than 50 trillion kW h systems manufacturer. Your team’s objective of power to run factories, machines, and is to build a solar-powered boat or car designed vehicles, and to heat all our buildings. for speed. You will need to research different Does that seem like a lot? It is not. designs and ways to build a solar-powered Every 40 min or so, the Sun delivers this model boat or car. Your team will also amount of energy free of charge to Earth’s develop a procedure to determine which surface. So why have we not made better use wavelengths of light produce more or less of this vast amount of solar energy? A few energy from the solar panel. decades ago, there was not much interest. Fossil fuels were cheaper to produce and use The Questions than the technology to collect solar energy. What design aspects will you need to Today, interest is growing rapidly, but there incorporate when designing a boat or a car remain difficulties to overcome. built for speed? What colour or wavelength Solar power is working its way into of light will produce the most energy for a many aspects of our lives. The most solar- powered model boat or car? common applications for solar technology in Saskatchewan are solar-powered lights, pool covers, recreational vehicles’ (RV) solar-powered equipment, and agriculture applications. Agriculture applications may include solar-powered electric fences, watering systems, lights, fans, and refrigeration. Commercial applications using solar technology are being supported through government Solar panels power this prototype solar vehicle. funding grants. The Confederation Inn in Saskatoon installed a solar-powered hot-water system. Residential homes are also starting to use solar panels to heat water and to provide electricity. Saskatchewan, with around 2200 h per year, is the province with the most sunlight year-round.
A solar-powered “Bottle Boat.” This boat uses empty drink containers for its hull and is powered by a flexible solar cell that can produce significant power under diffuse light conditions.
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c) developing a class presentation to Materials & Equipment include the information created and • drawing materials for making diagrams collected in the procedure. You might choose a poster, a computer slide • materials lists for building and testing your presentation, or a website display. solar-powered car (Lists are to be approved by your teacher.) Share and Compare
3 Present your design and the results of Procedure your procedure to your class. As you watch other teams present their designs, 1 a) Research different solar-powered write down: boat or car designs and the criteria a) what you liked best about their designs. for maximizing their speed. b) any ideas that you would like to use b) Draw a diagram of your solar-powered or change in your own design. boat or car. c) what you liked about their presentation. c) Create a materials list for your solar-powered boat or car. 4 Race your solar-powered boat or car d) Research and explain how a solar against other teams. Record your results. panel converts sunlight energy into Which team had the fastest boat/which electrical energy. the fastest car? e) Draw a schematic diagram of the circuit Observations and Reflections for your solar-powered boat or car. f) Develop a procedure to determine what 5 Prepare a report: wavelength of light will produce the most a) Which boat in the class was the energy in a solar-powered boat or car. fastest? which car? Why? g) Create a materials list for your procedure b) Which presentation was the most on the effects of wavelength on energy interesting? Why? production in your solar-powered c) What did you learn about the effects of boat or car. wavelength on solar-powered vehicles? h) Include any safety considerations in your procedures. i) Show your work to your teacher for approval.
Build
2 Carry out your procedures for: a) constructing your solar-powered boat or car. b) determining the effects of wavelength on energy production in your solar- powered boat or car. Which of these objects will you use in your project?
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UNIT SUMMARY
1.0 Light travels in straight lines and illuminance decreases with distance from its source.
KEY CONCEPTS SUMMARY
• light • Light moves in straight lines. • shadow • A shadow is created whenever light hits an opaque material. • illuminance • As light moves away from its source, the illuminance is less intense.
2.0 The law of reflection describes how light reflects from a plane mirror.
KEY CONCEPTS SUMMARY
• the law of reflection • The angle of incidence is equal to the angle of reflection. • concave mirrors • Concave mirrors are curved inward and show different types of images • convex mirrors depending on the distance between the object and the mirror. Concave mirrors can form both real and virtual images. • Convex mirrors are curved outward and form virtual images. • A ray diagram is used to represent how light travels.
3.0 Light is refracted by transparent materials, and this is what makes lenses so useful.
KEY CONCEPTS SUMMARY
• refraction • Light changes direction and speed as it passes through different materials. • lenses • A convex lens is used to focus light by converging light rays. • A concave lens is used to disperse light by diverging light rays.
4.0 The properties of light explain how the eye and the camera capture images.
KEY CONCEPTS SUMMARY
• human eye • Human eyes and cameras have many similarities, such as controlling the • camera amount of light entering the eye or the device, as well as the ability to • optical instruments focus the image. • Light enters the eye through the pupil and the lens focusses light onto the retina. • Lenses can correct problems with vision, such as near- and far-sightedness. • Lenses and mirrors can be used in combination to create optical instruments.
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The visible light spectrum is made up of different colours. Colours hold special meaning for 5.0 First Nations and Métis peoples.
KEY CONCEPTS SUMMARY
• visible light spectrum • Light can be refracted into the visible light spectrum. • meaning of colours • For First Nations and Métis peoples, colours have many deep, symbolic • addition model of colour meanings. These are often connected to the medicine wheel. • subtraction model of colour • The primary colours of light when mixed together form white light. • The colour of an object is due to colour subtraction. Objects absorb and reflect colours of light. The colours of light that are reflected are the colours we see.
6.0 Visible light is only one part of the electromagnetic spectrum.
KEY CONCEPTS SUMMARY
• wave model of light • The wave model of light explains all electromagnetic radiation. • properties of a wave • When wave motion is analyzed, the distance between the top of one • invisible spectrum wave and the top of the next is called the wavelength. • electromagnetic radiation • The electromagnetic radiation spectrum includes the spectrum of light • radiation technologies you can see, as well as other waves you cannot see. • natural or artifical light sources • The term used to refer to all forms of radiant energy is “electromagnetic radiation.” • The primary natural light source is the Sun. • Artificial light sources include phosphorescent and chemiluminescent sources; bioluminescent sources are a form of natural light source.
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UNIT REVIEW
Using Key Terms 4 a) Compare the human eye to a camera by using a table or a diagram. Include 1 Write a sentence that defines each of the the functions of each part. following terms: b) Use a ray drawing to demonstrate why the image formed on your retina is transparent upside down. Why don’t you see the opaque images upside down? reflection c) Which way does the image form on the umbra CCD matrix in a digital camera? penumbra specular reflection 5 Explain why you agree or disagree with diffuse reflection the following statements. For any you real image disagree with, provide the correct virtual image statement. incidence a) The normal is a line drawn at a refraction 90° angle to a mirror or lens. concave b) When light is reflected in a plane convex mirror, the angle of incidence is twice visible light spectrum the angle of reflection. electromagnetic radiation c) The visible spectrum includes infrared wavelength and ultraviolet light. frequency d) If you want to see farther into space, build a bigger mirror.
Reviewing the Big Ideas 6 What colour do you get when you mix the three primary-colour lights? the 2 Describe how you would appear in three secondary-colour lights? each of the following types of mirrors. Explain why. 7 What happens when light passes through a) a plane mirror a triangular prism? b) a concave mirror 8 How did First Nations and Métis peoples c) a convex mirror traditionally use colour? 3 a) What is a lens? 9 a) List the primary colours of light. b) What happens to light that passes b) List the secondary colours of light. through a concave lens? Why? c) How could you produce each of the c) What happens to light that passes secondary colours, using only the through a convex lens? Why? primary colours? d) Which type of lens would you find in a magnifying glass? 10 How does your eye detect colour?
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11 a) What is illuminance? 18 Why is the reduction of the ozone layer b) How is it affected by distance from the a problem? light source? c) Based on your answers above, explain the arrangement of plant lights and Connecting the Big Ideas basil plants in the photograph. 19 Why do you think stars twinkle when viewed on Earth but not in outer space?
20 A two-way mirror is designed to act as a mirror on one side and a transparent piece of glass on the other side. How do you think a two-way mirror works?
21 In colour printing, black is added to the three primary colours. Why do you think 12 a) Name two natural sources of this additional colour is needed? electromagnetic radiation. b) Name three artificial sources of electromagnetic radiation. Using the Big Ideas 13 Do waves with shorter wavelengths have 22 Some people living in the far north higher or lower energy than waves with experience depression and other similar longer wavelengths? illnesses during the winter. Research has 14 a) Compare an incandescent light bulb to shown this is partly due to the very short a compact fluorescent light bulb and amount of time the Sun shines during the to an LED light bulb. day. Which wavelengths of light would b) Which bulb would you pick for a you want to use in artificial lighting bedside lamp? Why? where people work and go to school in c) Which bulb would you pick to use in an these areas? Explain your reasoning. underground tunnel 5 km long? Why? 23 A reflecting telescope uses a concave mirror to gather light. 15 How do some animals make light, and a) Why is this mirror better for gathering what is the name of this phenomenon? light than a plane mirror? 16 What is the difference between b) Why is a telescope able to see more fluorescence and phosphorescence? images in outer space than it can on Earth? 17 In the Cree worldview, how are colours in the medicine wheel connected to symbolism for the stages of life? continued ᭤
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UNIT REVIEW (continued)
24 Suppose you are to design a searchlight 30 If a person is fishing with a bow and that produces a parallel beam of light. arrow where should they aim—above or The searchlight consists of a concave below the fish? If the person were using a mirror and a light. Where should the bulb laser gun where should they aim? Draw a be located along the principal axis? Draw diagram showing the two situations and a ray diagram and explain your reasoning. explain your reasoning.
25 Imagine that two people are stranded on 31 The Canadian government has proposed an island and both are wearing glasses. that incandescent bulbs will be phased One person is farsighted and the other is out for common applications by 2012. nearsighted. If they needed to start a fire, Incandescent bulbs have an efficiency of which person's glasses would be useful 2%, compact fluorescent bulbs about 22%, in starting the fire? Where should the and LED bulbs about 22%. Why does the wood shavings be placed so the fire can incandescent bulb get hotter than LEDs or be started as quickly as possible? Draw a fluorescent light bulbs? Why don’t people ray diagram and explain your reasoning. use compact fluorescent bulbs or LEDs? What factors might affect their decision? 26 Two rays of light converge to a focal point
on a screen. A plate of glass is placed 32 A declaration was signed between the parallel to the screen and directly in the provinces of Saskatchewan and Alberta and path of the two rays of light. Will the the Government of Canada in partnership focal point of the light rays remain on the with the Royal Astronomical Society screen? If not, will the focal point move of Canada to designate Cypress Hills in front or behind the screen? Draw a ray Interprovincial Park as a dark-sky preserve. diagram and explain the location of the Why do you think this declaration is light ray's focal point. necessary? Would you support such a 27 Imagine a situation where the word declaration in your community? SASKATCHEWAN has the following letters (from left to right) written in Magenta SACHWAN. The other letters SKT are in Self Assessment Blue and AE are in Red. The background is 33 What are the key ideas of this chapter? white. What will you see when you shine green light on the word SASKATCHEWAN? 34 Which of the key ideas did you already know before you started this chapter? 28 Why are images reflected in a window seen more easily at night from inside the 35 Which of the key ideas did you learn house, whereas during the day they are during the course of this chapter? not seen? Explain your reasoning. 36 Which of the key ideas do you still have 29 If you wanted to make a rainbow by questions about? How will you find the spraying water into the air from a garden answers to those questions? hose, where must you stand relative to the Sun and the water to see the rainbow? 37 What is one idea or fact that amazed you Is it possible to walk under a rainbow? during the course of this chapter? Explain your answer.
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