Light and Optical Systems
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UNIT 3 Light and Optical Systems What is light? You know it allows you to see, but do you know how? Do you know what causes the shimmering colours in the northern lights shown here? Why do we see in colour? For centuries, scientists around the world have tried to find answers to questions such as these. They developed models to try to explain their observations about light. Some early scientists thought that light might consist of particles that enter our eyes and make us see. However, when light shines on an object, the object’s mass does not increase. Therefore, light must be a form of energy, but what form? After hundreds of years of observations and experiments, scientists have not found one simple model that explains all of the characteristics of light. One helpful model is the ray model. A ray is an imaginary line that represents the direc- tion in which light energy travels. While it explains how light forms images in a mirror and shadows on the wall, the ray model cannot explain colours or the way that light behaves when it travels through a very tiny slit. The wave model explains these characteristics. In this unit, you will learn how to use these models to develop an understanding of how light works. You will also learn how we can use light technologies to assist us in everyday life. For example, an understanding of light has made it possible to develop advanced technologies such as lasers (used in eye surgery) and fibre optics (used in communications). 172 Unit Contents TOPIC 1 What Is Light? 176 TOPIC 2 Reflection 188 TOPIC 3 Refraction 200 TOPIC 4 Lenses and Vision 208 TOPIC 5 Extending Human Vision 221 TOPIC 6 The Source of Colours 228 TOPIC 7 The Wave Model of Light 237 TOPIC 8 Beyond Light 249 UNIT 3 In how many different ways have you used • What is light? light today? How often have you used a mirror • What inventions use or seen reflections off a shiny surface? In light? Topics 1–3, you will learn just what light is and how it behaves in • What do these inventions predictable reveal about the nature ways. of light? We have opened up the universe to our eyes. How? With the help of powerful telescopes, astronomers can study the birth of stars, such as the Orion Nebula shown here. In Topics 4–5, you will investigate some fantastic technologies that extend human vision in ways unimaginable even a few decades ago. 174 MHR • Light and Optical Systems What do you think is causing the bands of colour in these photographs? What exactly is colour? Do you know what a spectrum is? In Topics 6–8, you will find out, if you don’t yet know… Read pages 260–261, “Design Your Own Investigation.” Here is a chance to display your skills in designing controlled experiments. You can start planning your investigation well in advance… Start sharing ideas with your teammates. Save all your ideas in an “Experiment Planning File.” Begin collecting the materials you will need for your experiment. Think about how you might present your experimental results in a multimedia presentation. Unit 3 Preview • MHR 175 TOPIC 1 What Is Light? In the simplest terms, light is the form of energy that you can see. The Sun is a natural light source. It is the source of the most abundant and least expensive light in the world. Fire is another natural source of light (see Figure 3.1). Light from the Sun is The Sun is a star; all stars in the universe are sources of light. Light produced by nuclear spreads out, or radiates, from the Sun and other stars, in all directions, fusion of hydrogen like the spokes of a bicycle wheel. This type of energy transfer does particles. The Sun is not require matter; it is known as radiation. Energy such as light that composed of about travels by radiation is often called radiant energy. 70 percent hydrogen. When hydrogen Less than one tenth of one millionth of a percent of the Sun’s energy particles fuse (combine), actually reaches Earth. Nonetheless, our lives are totally dependent they form another upon this energy. Plants, people, and other animals could not live with- particle called helium. out light from the Sun. Because sunlight is not always available, people During this process, an have developed light-producing technologies, or artificial lights. A light enormous amount of bulb is an example of an artificial light source. Like the Sun, light energy is released as heat and light. The from a bulb radiates in all directions. temperature inside What else can produce light? Think about what happens when you the Sun is about strike a match. Chemicals on the tip of the match react to produce heat 16 000 000˚C! and light. Once the chemical energy is used up, the match is no longer useful. Like the match, all other sources of light require energy. Flashlights use electrical energy from batteries. Light bulbs glow when you switch on electricity. The light that leaves the Sun is formed through a process called nuclear fusion. Look around the room. List all the items you can that either give off or reflect light. You will discover a great deal about how light behaves as you study this unit. Figure 3.1 Besides the Sun and the stars, flames and sparks are natural sources of light. 176 MHR • Light and Optical Systems Find Out Is Light Energetic? In the introduction, you read that light is a form 2. What change did you notice about the of energy. Scientists often tell us that energy canister after it had been exposed to a has no matter but you can tell that it exists bright light? because it can cause changes in matter. Can Analyzing and Interpreting you use this concept to show that light is a 3. What evidence do form of energy? you have that light caused a change in both steps 1 and 2 of the procedure? Materials Extensions solar-powered calculator 2 identical black film canisters 4. When light energy is absorbed by solar aluminum foil cells, into what form of energy does it bright light source, such as a 100 W bulb change so that the calculator can use the energy? Procedure 5. In your notebook, complete the following 1. Find the solar cells on a calculator. Enter sentence: “Light can be changed into some digits, then completely cover the energy forms such as …” solar cells with your finger to block the light. Observe what happens to the digits. 6. In step 2 of the procedure, what is the manipulated variable? What is the 2. Feel the inside wall of the canister. Replace responding variable? What variables the lid of the canister and place it in a should be controlled to obtain bright light, such as sunlight or light from a meaningful results? 100 W bulb. Wait a few minutes, then 7. Repeat procedure remove the lid of the canister and feel the step 2 with two canis- inside surface. ters. Before placing What Did You Find Out? the canisters in a 1. What happened to the digits when you bright light, cover one prevented the light from reaching the solar of the canisters with cells on the calculator? Some solar calcula- aluminum foil (shiny tors have a second source of power, such side out). What can as a battery. What can you infer from your you infer about the observations, about the power source of effect of the foil? the calculator you tested? Satellites use solar cells The First Basic Principle of Light to power their electronic equipment. Someday, we You have seen that light is a form of energy. This is the first basic might all use sunlight to principle of light. When light is absorbed by a surface, it can be trans- produce the electrical formed into several different forms of energy. Light can be transformed energy we need. In 1987, into thermal energy, electrical energy, or chemical energy. For example, the Sunraycer, a test car the absorption of sunlight by a black sweater causes the garment to covered with solar cell gain thermal energy. Solar cells change light into electricity. Trees in panels, drove across Australia powered only your neighbourhood absorb sunlight to make chemical energy (sugars). by energy from the Sun. What is Light? • MHR 177 Figure 3.2 Sunlight is The brightness, or intensity, of light absorbed by the pavement indicates how much energy a surface on this runway and transformed into thermal will receive. A surface can absorb energy. You can see the more energy if the brightness of the effect of heated air rising light intensifies. For instance, pave- from the pavement on a ment may feel hot to the touch on a road or a runway on a hot, sunny summer day. sunny summer day (see Figure 3.2). However, the pavement will feel only warm if the clouds block out the sunlight. In the activity below, explore further the concepts of light, intensity, and radiant energy. Find Out Reading with Intensity Light intensity is determined by how much 4. Repeat steps 1 to 3 using a 100 W bulb. energy is received on a surface. In this activity, you will observe how distance affects the What Did You Find Out? Analyzing and Interpreting intensity (brightness) of light striking an object. 1. How does increasing the distance from the bulb affect the intensity of the light striking Safety Precautions the book’s pages? 2.