В2 Additive Color Model and Vision

INVESTIGATION B2 Collaborative Learning This investigation is Exploros-enabled for tablets. See page xiii for details.

B2 Additive Color Model and Vision Key Question: How do we see color?

In this investigation, students explore what happens Online Resources when they mix different colors of light. They use Available at curiosityplace.com flashlights with color filters to project colors of light onto yy E quipment Video: Optics with Light & Color a screen. Then, they observe the result when they mix different combinations of the three primary colors of yy Skill and Practice Sheets light: red, green, and blue. They apply what they learned yy Whiteboard Resources to human vision and how we perceive color. Finally, yy Animation: Additive Color Model they use diffraction glasses to observe light from the yy Science Content Video: RGB Color Model flashlights. This allows them to see that white light is made of all of the colors of light. yy Student Reading: Vision and Color Learning Goals Vocabulary additive color model – a process that creates color by ✔ Use flashlights to mix primary colors of light and show ✔ adding proportions of red, green, and blue light together that white light can be made from red, green, and color – a property of visible light that is related to its blue light. wavelength ✔✔Compare sources of light. cone cells – photoreceptors on the surface of the retina ✔✔Explain how humans see color. that respond to color diffraction grating – an optical device consisting of an assembly of parallel narrow slits or grooves that interfere GETTING STARTED with incident radiation to disperse light waves, and can result in spectra Time 50 minutes photoreceptors – light-sensitive cells on the surface of Setup and Materials the retina of the eye pixel – the smallest element in a display or image 1. Make copies of investigation sheets for students. rod cells – photoreceptor cells in the retina of the eye 2. Watch the equipment video. that respond to differences in brightness 3. Review all safety procedures with students. visible light – the light you can see in the range between Materials for each group 400 and 700 nanometers white light – light containing an equal mix of all colors yy Optics with Light & Color kit

NGSS Connection This investigation builds conceptual understanding and skills for the following performance expectation. HS-PS4-3. Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.

Science and Engineering Practices Disciplinary Core Ideas Crosscutting Concepts

Engaging in Argument from Evidence PS4.A: Wave Properties Systems and System Models PS4.B: Electromagnetic Radiation

Optics with Light and Color 51 Additive Color Model and Vision

BACKGROUND The human eye also contains another type of light- sensing cell called a rod cell. Rod cells sense the overall intensity of light and therefore see in black and white. Every time humans see something, light is involved. Like They do not see color. Because they can respond to all heat and sound, light is a form of energy. Light comes colors, rod cells are much more sensitive, and can detect to our eyes in two ways: directly from a light-producing lower levels of light, than cone cells. This is why colors object, like a star, or reflected from objects that do not seem washed out in the dark. The lower the overall level produce their own light, like the paper page of a book. of light, the more the eye sees only black and white When we see things in good light, we usually perceive a images. There are about 130 million rod cells and only color. The sky tends to be blue in the middle of the day, about 7 million cone cells. This means the sharpness in or it might have reds and oranges in the evening. A fire our vision comes mostly from our perception of black alarm is usually red; things like walls and shirts may be and white. The color associated with each triplet of any color including black and white. three cone cells is associated with the brightness seen White light is a combination of all the colors of visible by 60–100 rod cells. Essentially, the image that you see is light. A dispersive element like a prism or a diffraction assembled in your brain from 130 million black and white grating interferes with light in a way that can separate dots and 7 million colored dots. out the colors to show that white light is in fact made In this investigation, students discover what happens up of many colors. Simply speaking, these dispersive when different colors of light are mixed. We focus on the elements use refraction or diffraction to bend or change additive color model because we are generally talking the path of light. Light and other waves of different about emitted light and transmitted light. By combining wavelengths respond differently to these conditions, colors of light, we are adding light energies together and essentially causing them to be separated out. If you have therefore sending more light to our eyes in the areas seen light spectra from prisms or spectrometers, you are that perceive those colors. We use the color filters in this seeing refracted or diffracted light, respectively. investigation create the three primary colors of light: red, Since light is energy, different colors are simply lights green and blue. We can then combine light from two or of different energies. Color is how humans perceive more flashlights to demonstrate the additive model. the energy of light. When we talk about colors of light, Televisions and computer monitors make red, green, we talk about wavelength. The double-slit experiment and blue (RGB) light directly (graphic below). Use a conducted by Thomas Young (1773-1829) at the magnifying glass to look closely at a white area on a TV beginning of the 19th century is the earliest published screen or computer monitor and you will see that what experiment using diffraction gratings to show that light appears white is actually tiny red, green, and blue dots. must be made up of waves, and further, that different The dots are called pixels and each pixel gives off its colors have different wavelengths and frequencies. own light. The pixels are separated by very thin black All the colors of visible light can be created using lines. The black lines help give intensity to the colors and combinations of three primary colors: red, blue, and help make the dark colors appear darker. From far away, green. While we could use other colors of light, we you cannot see the individual pixels. Instead, you see a generally use red, blue and green because we have nice, smooth color picture. By turning on the different photoreceptors in the eye called cone cells that are color pixels at different “tuned” to these three colors. We use the letters from the intensities, TV sets can mix colors, RGB, to describe this model of color creation and the three primary colors perception. While most of us think that color is inherent to get millions of different in all objects, an object’s color is really just a way we colors. For example, a perceive light of different energies or wavelengths that light brown color could be are reflected or transmitted to our eyes. displayed by illuminating 88 percent of the red, 85 percent of the green, and 70 percent of the blue pixels. 52 B2

5E LESSON PLAN

Engage Elaborate Turn out the lights and turn on a flashlight with no color Making Colored Shadows Using Additive Color Mixing filter. Ask the students what they see. [White light.] If they don’t describe the light, ask them to. Next, have them all When an object is placed in the path of light, it is made put on diffraction grating glasses and look at the light visible if you are able to view it from the side of the light. again. Ask them once again to describe what they see. Ask But light also casts a shadow directly behind the object. them what they think happened to the light. In making shadows, the object in the path of light is called the occluding object. When there is more than one When they put on the glasses, they should see an array of source of light, multiple shadows can form. The regions dashes of color in red, orange, yellow, green, and blue. This where some light gets by blocked are called penumbra. demonstration will prepare them for understanding the The region where no light shines is called the umbra. idea that the rainbow of colors came from the white light. Demonstrate this using the following procedure: Light A Explore Penumbra Have students complete Investigation B2, Additive Color B Model and Vision. In this investigation, students view light Umbra sources in three primary colors. They see how mixing colors in different combinations creates new colors of Penumbra light. They discuss different sources of light that we use A every day, and learn how we see the light and images Light B all around us. Students will learn how we might see the 1. Connect the three flashlight holders in a line using the same color created by two different methods. Finally, slots and rails, arranging all three holders side by side. they see how filters work to create color. 2. Position the flashlights in the holders with the blue light in the middle on the laminated grid. Align the Explain flashlights with their ends aligned with the edge of Revisit the Key Question to give students an opportunity the grid. to reflect on their learning experience and verbalize 3. Fold a clean sheet of letter-sized paper in half and understandings about the science concepts explored in tuck half under the back side of the closed box to the investigation. Curiosityplace.com resources, including prop it up as a screen at the other end of the grid. student readings, videos, animations, and whiteboard 4. Turn off the classroom lights; the room should be as resources, as well as readings from your current dark as possible. Turn on the flashlights. science textbook, are other tools to facilitate student communication about new ideas. 5. Place the laser in line with the center flashlight so that it stands up like a pole about 10 cm (20 squares on the grid) from the front of the flashlights, as shown in the diagram below. (continued on next page)

Science Content Video Animation RGB Color Model Additive Color Model

Optics with Light and Color 53 Additive Color Model and Vision

Ask students to draw and describe the projected image. Ask students how they can prove your explanation by [Students should see that there are three vertical bands turning on only one light at a time. They might surmise of color. Outside the bands is a whitish light. There are that if you turn off all but one light at a time, there is a two small overlapping sections at the base of the bands.] single shadow formed for each light. For each flashlight, the position of the shadow is the location of the band Ask students, “How many different colors of light do of color formed by the other two color lights. So, for you see? What are they and where are they?” Then have example, the blue light in the middle forms a shadow students make guesses as to how the colors were made. in the middle. That shadow is where green and red combine to form yellow. cyan yellow magenta Finally, ask students, with all the lights on, what are the areas of color called, umbra or penumbra? The answer is penumbra. Umbra is where no light shines. There are no places where there is umbra from the pole.

whitish whitish Evaluate yy During the investigation, use the checkpoint questions as opportunities for ongoing assessment. yy After completing the investigation, have students

green red answer the assessment questions on the Evaluate student sheet to check understanding of the The three colors combine to make the whitish light concepts presented. that surrounds the bands. The band shapes match the shape of the laser-pole. Since there are three distinct light sources, the pole casts one shadow from each light source, but light from the other lights shines there and creates a penumbra. For each large band, the pole blocks light from one light source, and the two unblocked colors are mixing to create a new color: cyan from blue and green, yellow from green and red, and magenta from red and blue. In the smaller regions where we see green and red, there is a “double” overlap of shadows. Two light sources are blocked. There, you have one pure color from each of the two outside light sources. In one of the smaller sections, blue and red are blocked, showing green. In the other small section, blue and green are blocked, showing red.

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Explore INVESTIGATION B2 Explore INVESTIGATION B2

Name ______Date ______2. Put a different color filter on each of the three flashlights. Slide all three flashlights into the three holders so that blue is on the top and green and red below. B2 Additive Color Model and Vision Materials: Optics with Light & Color kit: How do we see color? • 3 Flashlights with holders All the colors of visible light can be created artificially using a • Red, green, and blue color filters combination of three primary colors: red, blue, and green. Don’t believe • Laser flashlight it? You will use a white light source and color filters to discover what happens when you mix different colors of light. You will also learn how • Convex lens with light blue those filters work with your eye to create the impression of different holder colors in your brain. • Laminated grid 3. Position the flashlights on one side of the laminated grid. • Diffraction grating glasses Safety Avoid shining a laser or flashlights directly into the eye. 4. Set the white box that the Optics with Light & Color kit comes in at the other edge of the laminated grid. Open the lid straight up to function as a screen for reflecting the lights.

5. Turn the flashlights on.

6. Position the light blue lens with the split side facing down between the flashlights so the lights shine  Mixing primary colors of light through it onto the box. Slide it toward or away from the box until you see three sharp spots of color First, let’s explore what happens when you mix the primary colors of light: red, green, and blue. Follow the overlapping each other on the box. procedures below.

1. Connect two of the flashlight holders using the rail and slot connectors on the side. Leave the third holder unconnected. Place the third connector on top of the two connectors, making a small pyramid stack.

pyramid slide holders together stack slot rail

Turning off the lights will help you to better see the images on the screen.

Copyright © CPO Science B2 Additive Color Model and Vision Copyright © CPO Science B2 Additive Color Model and Vision Can be duplicated for classroom use 1 of 7 Optics with Light and Color Can be duplicated for classroom use 2 of 7 Optics with Light and Color

Guiding the INVESTIGATION

 Mixing primary colors of light This photograph shows the proper setup for Part 1 of the investigation.

use lens to focus flashlights with Convex lens in light here color filter caps light blue holder

Laminated table

position holder with Box split side down

Optics with Light and Color 55 Additive Color Model and Vision

ADDRESSING MISCONCEPTIONS Explore INVESTIGATION B2

 Explaining what you see In the investigation, students mix colors of light Follow the steps and use Table 1 to record the answers to the following questions about your observations.

and learn about the additive color model. This a. Turn on just the blue and red flashlights. What color do you see when you mix blue and red light?

process is different than mixing paints or pigments, b. Turn on just the green and blue flashlights. What color do you see when you mix green and blue light?

which employs the subtractive color model. When c. Turn on just the red and green flashlights. What color do you see when you mix red and green light?

students mix red d. Turn on all three flashlights. What color is produced when all three colors of light are equally mixed? and green light to Table 1: Mixing primary colors of light get yellow, most are surprised by this Color combination Color you see result. The same is blue + red pink (magenta)

true when the three green + blue sky blue (cyan) colors mix to achieve red + green yellow white light. You can’t achieve anything like red + green + blue white that with paints and  Sources of light dyes. Additive color a. Televisions, computer monitors, and cell phone display screens use dots of light to create images. These is about adding light, dots are called pixels. What colors of light can these devices use to make all the possible colors needed for the images they display? not pigment. Mixing All they need are red, green, and blue. non-white dyes on paper, for instance, tends to darken colors because more colored pigment

TEACHING TIP Guiding the INVESTIGATION

When setting up the color flashlight activity, it is a  Explaining what you see good idea to use the laminated grid as a positioning Tell the students they are looking for where the guide and the white box for a screen. Position the colors overlap each other. Let students share guesses. flashlights, lens, and box as shown in Part 1 of the After they mix the colors, use the questions in Part 2 investigation for an ideal projection and mixing to lead a discussion about students’ findings. Mixing of colors. You can also use a folded piece of white red and blue produces the color magenta. When paper to make a screen. blue and green are mixed, the resulting color is light blue (cyan). Mixing red and green creates yellow. When all three colors of light are equally mixed, white light is created. This will help students see clearly that white light is a mixture of many colors. If the students can get three circles of equal intensity to overlap the screen, what they will see is a pretty clear white spot in the area where the red, green, and blue circles overlap.

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Guiding the INVESTIGATION Explore INVESTIGATION B2

b. How do you think they use these colors to create images?  Sources of light They use pixels in the colors of red, green, and blue to create regions of color that when seen together create an overall image. Tell students, “Television screens use the three

primary colors of light to create the images humans c. Look at your clothes. Does the light reaching your eyes from your clothes originate in your clothes, or see. If you examine a television with a magnifying does it come from somewhere else? The light does not come from my clothes; it comes from lights and glass, depending on the resolution of the screen, the sun. you will see that the screen is made of tiny red,

green, and blue dots of light. By turning on the tiny d. What color would your clothes appear to be in a room with no light? lights at different intensities and times, television The clothes can’t be seen in the dark. They are definitely not screens can mix these three colors to make millions producing their own light, so they would appear to have no color at of different colors. From far away, your eyes cannot all, and all I would see is black. see the tiny dots, but instead blend them together to see a nice clear picture.” Demonstrate this with a  Researching how we see colors classroom television (or other electronic display) and magnifying glasses if they are available.

Prompt students to steer the discussion of red, blue, and green “dots” toward pixels. Most students will relate the word pixel to the resolution of their digital cameras or their mobile phone cameras. Allow a few student volunteers to discuss how increasing the resolution of the camera produces a better quality image.

Copyright © CPO Science B2 Additive Color Model and Vision 4 of 7 You can also start a discussion about why objects Can be duplicated for classroom use Optics with Light and Color appear a certain color. A shirt looks blue because it reflects blue light, which means there must be blue light in whatever light falls on that shirt to be reflected. Turn off the lights and shine a red light on a blue shirt. Students should observe that the blue shirt looks black.

The students can also demonstrate this concept themselves. Turn off the classroom lights and have them cup the laser in their hand, shielding out light where the red laser label is. With the green filter on a flashlight, shine the green light on the label. The letters on the label will appear green but the background of the label is black. This is because there is almost no red in the green light, and thus no red is reflected to the eye.

Blue cloth looks blue because there are dyes in the cloth that absorb all colors of light except blue. When white light falls on the fabric, everything except blue is absorbed. Since blue light is what is reflected to your eye, the shirt looks blue.

Optics with Light and Color 57 Additive Color Model and Vision

Explore INVESTIGATION B2 Explore INVESTIGATION B2

a. Research and explain the following terms from the diagram above: cone cells, rod cells, retina, and optic d. Research and explain how the eye sees white light in terms of the photoreceptors in the eye. nerve. Which cells are responsible for sensing color? White light stimulates the three kinds of cone cells. The red, green, Cone cells – photoreceptor, or light sensing, cells in the retina of and blue cone cells send signals to the brain via the optic nerve the eye that respond to color indicating all three colors are present in a particular area, and the brain sees white in that area. Rod cells - photoreceptor cells in the retina of the eye that respond to differences in brightness  Examining color filters A red laser can produce a single pure red light that can be useful in some applications. For example, making three-dimensional images called holograms requires the very pure source of light that a laser can provide. Retina - section of light-sensitive tissue at the back of the eye onto In the Optics with Light & Color kit, your flashlights with filters give you three different sources of colored which images are focused by the lens of the eye light, but just how “pure” are these colors? In this part of the investigation, you will examine the light produced by each flashlight and learn how a color filter works.

Optic nerve - The nerve that serves to transmit information from 1. Put on a pair of diffraction grating glasses. the photoreceptors in your eye to your brain 2. Turn on a flashlight. Tilting it only slightly toward your eyes, observe the pattern you see.

b. Research and explain how the eye sees green light in terms of the photoreceptors in the eye. 3. Put each of the color filter caps on the flashlight and observe the colors you see for each.

When green light is detected by green cone cells in a particular area a. What colors did you see with the white light? Which colors seemed strongest? in the eye, signals are sent down the optic nerve to the brain that Violet, blue, green, yellow, orange, red. Blue, green, and red seem to green is being detected. The brain then sees green in that area. be the predominant colors.

c. Research and explain how the eye sees yellow light in terms of the photoreceptors in the eye. b. Why do you think you saw all these colors? There are two ways yellow can be seen. One way is for yellow light The white light from the flashlight is made up of light of many to be detected by photoreceptors in the retina. Both red and green different colors. cone cells are sensitive to yellow light. When both are stimulated in a particular area, they both send a signal to the brain via the optic c. What colors did you see with the colored caps? nerve, and yellow is seen in that area. Blue filter cap – predominantly blue, some red and a little green Another way is for green and red light to enter the eye together in Green filter cap - predominantly green, some blue and very little red a particular area, which results in both red and green cone cells and yellow sending signals to the brain via the optic nerve, and the brain Red filter cap - predominantly red, with yellow, green, and blue interprets this as yellow in that area.

B2 Additive Color Model and Vision Copyright © CPO Science B2 Additive Color Model and Vision Copyright © CPO Science Can be duplicated for classroom use 5 of 7 Optics with Light and Color Can be duplicated for classroom use 6 of 7 Optics with Light and Color

Guiding the INVESTIGATION color do you see?” Students should answer green. We see green when the brain gets signals from cone cells  Researching how we see colors which respond to medium energy light. Tell students, “The experiments you completed today Then ask, “What happens if the energy of light is are as much about human perception as they are about between low and medium? The low energy cone physics. For example, when you mixed green light and cells and the medium energy cone cells both send red light you saw yellow. You probably did not expect signals to the brain. What color do you see?” Students to see yellow because yellow is a light color and red learned from the and green seem like darker colors. You see yellow experiment that because of how the human eye and brain detect and we see a color perceive light. The retina of the eye has three types that we call yellow of color sensors called cone cells. Each type of cone when there is an cell is receptive to a particular energy of light. When equally strong high energy light is received by the eye, only the blue signal from both cone cells send signals to the brain. We have learned the red cone cells to associate these signals with the name blue. Suppose and the green the cone cells for medium energy are active. What cone cells.

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Guiding the INVESTIGATION Explore INVESTIGATION B2

d. Did you see the color red when you viewed the different filtered light through the diffraction grating glasses? Rank the filters according the amount of red you saw with each, with 1 being the most red  Examining color filters and 3 being the least. 1-red filter, 2-blue filter, 3-green filter

Explain to students that a film like the one in the 4. Turn on the laser, shining it at an angle toward the ceiling. Observe the light with the diffraction grating diffraction grating glasses is a transparent material glasses. with a series of ridges or lines that cause light a. What colors do you see? Only red.

to bend or diffract. Different colors of light have 5. Now put the red filter on the laser, aiming the light at the palm of your other hand a few inches away. different wavelengths. The effect of diffracted light Repeat this with the blue and green filters. is different for different wavelengths, so you can see a. Describe what happened with each filter. The red filter allowed the red laser light to reach my palm clearly. the different wavelengths that make up the light. The blue filter allowed some of the red laser light to reach my palm, but faintly. The green filter allowed no light to reach my palm. Using a diffraction grating, it is easy to show that

white light is composed of many colors. Demonstrate b. Based upon your experience with the laser and the filters, explain why we call them filters. this point if time permits. Have students pick up the The filters allow only some light through them. Some light is diffraction grating glasses and look at a light source absorbed or blocked. (such as one of the flashlights) through them. They c. Based upon your experience with the white light and the diffraction grating, explain what you saw should see a central bright spot and rainbows up, with the laser and the filters. down, and to the side. The glasses separate light into The white light is made up of all colors, which you can see when we disperse the light with the diffraction grating glasses. When you a spectrum that shows the component colors in the apply the filters, only certain colors are allowed through. Red filters rainbows on the sides of the central bright spot. let the most red through and green filters let the least.

The color filter works by removing colors from light. If a light source has only one wavelength, it has only If students use the diffraction grating glasses to look one color. The laser is a perfect example of this kind of at each of the lights with color filters, they should light. The students put each filter on the red laser and observe that each of the colored lamps has a spectrum. aim it at the palms of their hands. The red filter lets the A light looks green if green is the dominant color, even most light through and shines a bright beam on the if the light includes other colors as well, as with the palm. The blue will shine a faint light. The green, which green light students produce in this investigation. For showed the least red using the diffraction grating and example, the red lamp has mostly red, but it also has the flashlight, will essentially block the red light and no some green and a blue. The blue lamp has mostly blue red light should appear to shine on their palms. but it also has a small amount of red and even some green. The green has a little blue but almost no red. Have students compare the spectrum they see from the red, green, and blue lights with each other. They will rank the filtered light by which one has the most red and which one has the least.

Optics with Light and Color 59 Additive Color Model and Vision

Notes and Reflections Evaluate INVESTIGATION B2

Name ______Date ______

1. In this lab, you mixed colored lights together to get light that appears different. Match the combination on the left with the resulting color on the right. green + red magenta/pink blue + green yellow blue + red white blue + red + green cyan/light blue green + red make yellow blue + green make cyan/light blue blue + red make magenta/pink blue + red + green make white

2. What is the process of mixing colored light called? Mixing primary colors of light to achieve all the colors of visible light is called the additive color model.

3. What are the cell types in human eyes that perceive color? Red cone cells, green cone cells, and blue cone cells.

4. Describe two ways we might perceive light in the color cyan (light blue)? One way is for the color to be from a single source. It enters the eye and is detected by our blue and green cone cells. Another way is for green and blue lights to combine in the same area. They both enter the eye together and are detected by the blue and green cone cells as coming from the same area, creating the perception of cyan.

5. A diffraction grating is a good way to disperse light. Explain how this might confirm what is happening when you add different colors of light together to make white light. When we look at white light through the diffraction grating glasses, we can see that the light is made up of many colors including red, orange, yellow, green and blue. If we were to combine light in these colors, we would get a white or whitish light.

WRAPPING UP

Have students reflect on what they learned from the investigation by answering the following question:

Write a paragraph that summarizes how we perceive the colors of light. Use vocabulary you learned in this investigation in your paragraph.