COLOR. Diandra Leslie-Pelecky Edited by Anne Starace. Abstract. Keywords: color mixing, additive color mixing, subtractive color mixing

COLOR Diandra Leslie-Pelecky Edited by Anne Starace Abstract The perception of color is a fascinating phenomenon with its origin in the behavior of light. Using the Singerman color apparatus and color filters, students will learn about additive and subtractive color mixing. Keywords: color mixing, additive color mixing, subtractive color mixing Funded by the National Science Foundation and the University of Nebraska

Content Standards K 1 2 3 4 5 6 7 8 1.2.1 4.2.1 1.3.1 4.3.1 4.3.3 History & Process Standards K 1 2 3 4 5 6 7 8 Skills Used/Developed: 2002 University of Nebraska. 2

TABLE OF CONTENTS I. OBJECTIVES...4 II. SAFTEY...4 III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS...4 IV. REQUIREMENTS...4 V. INTRODUCTION...5 What is Light?...5 The Visible Range...6 Color and Radiation...7 Emission...7 Absorption, reflection and transmission...8 Color Addition...8 Color Subtraction...8 How Does the Eye See Color?...10 VI. PROCEDURE...11 VII. FREQUENTLY ASKED QUESTIONS...13 VIII.TROUBLE SHOOTING...13 IX. HANDOUT MASTERS...13 X. REFERENCES...13 2002 University of Nebraska. 3

I. OBJECTIVES Students will: -observe additive and subtractive color mixing. -learn about the electromagnetic spectrum and understand that humans can only see a small part of it. -understand that different colors of light have different wavelengths. II. SAFETY -Turn down the intensities on the bulbs when changing filters and when you re not using the apparatus. It gets hot very quickly. -The ground-glass screen is fragile. Handle carefully. -The bulbs get hot. Don t touch them while aligning. III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS LEVEL The activity is aimed at elementary school children (and their parents) TIME REQURED The activity takes about 20 minutes. NUMBER OF PARTICIPANTS 3-20 IV. REQUIREMENTS A. Materials Singerman Color Apparatus Computer (if using accompanying powerpoint) Stubby slot screwdriver Color paddles Three subtractive color disks in cyan, magenta and yellow. Spare bulbs (need carrier) Large prism B. Facilities Large white screen 2002 University of Nebraska. 4

400 nm 500 nm 600 nm 700 nm Infrared X-rays Ultraviolet Microwave Gamma rays E H F S H F U H F V H F H F M F L F ELF 0.001 0.01 0.1 1 10 100 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 λ (nm) Figure 1: The Electromagnetic Spectrum V. INTRODUCTION You ve probably heard that light sometimes behaves like waves and sometimes like particles. How we describe light depends on what type of phenomena we want to describe. In some cases, it will be convenient for us to think of light as a wave. In other cases, we can think of light as a beam of particles. What is Light? Light is electromagnetic radiation. Light waves move at a speed of 3 x 10 8 m/s in a vacuum. As far as we know, nothing can move faster than the speed of light. Although electromagnetic radiation spans a very broad range of wavelengths, the portion of electromagnetic radiation that we can see is only a small part of the entire range. The electromagnetic spectrum is shown in Figure 1. In addition to the visible regime, we have: Gamma rays are very short wavelength radiation and correspond to a small packet of electromagnetic energy. Gamma rays are very penetrating and can pass through materials that would normally shield x-rays. X-Rays are the next shortest wavelength radiation. We have just celebrated the 100 th anniversary of the discovery of x-rays by William Roentgen. X-Rays are produced when electrons hit an atom and interact with the electrons in the atom. This interaction is repulsive and causes the incident electrons to slow down. When they slow down, they radiate electromagnetic energy and it is this energy that is called x-rays. X-Rays are very high frequency and thus are very high energy. They can cause cancer, skin burns and other effects; however, if used at low intensities, x-rays can be used relatively safely to image the internal structure of the body. Dense objects absorb more x-rays, so bone will absorb more x-rays than tissue. By exposing a photographic plate, the 2002 University of Nebraska. 5

absorbing regions block the x-rays from hitting the plate, thus providing an image of the internal structure of the body. UV, or Ultraviolet Radiation is at the violet end of the visible spectrum. UV light is produced by very hot objects, including the sun. Most of the UV radiation emitted by the sun is absorbed by the ozone layer. This absorption is very important, because UV radiation can cause your skin to burn. The pigment in your skin melanin also absorbs UV rays to protect your skin. Melanin is brown and exposure to sunlight causes you to produce more melanin up to a point. Overexposure causes sunburn and can damage cells, producing skin cancer. On the positive side, exposure to sunlight is necessary because it produces vitamin D, which is essential for strong bones and teeth. Because people who live in climates where it isn t sunny all the time have limited exposure to the sun, vitamin D is often supplemented in milk. Some glasses and plastics can absorb UV radiation and thus offer protection to your skin and your eyes. Welding torches give off lots of UV radiation and the special welding goggles used are in part to protect the eyes from the UV radiation. Blacklights emit radiation in the violet and near UV. A phenomenon called fluorescence is the process by which a material absorbs UV light and emits visible light. This is what causes clothing to glow under blacklights. Skipping over visible light for a moment, we have next IR, or infrared radiation, is adjacent to the low-frequency or long-wavelength end of the visible spectrum. A body at about room temperature emits radiation in the far infrared region. Water molecules readily absorb infrared radiation, so IR lamps are often used as heat lamps to keep food warm in cafeterias, or to warm up bathrooms. Microwaves have frequencies in the GHz and are used in communications and radar applications. Like infrared radiation, microwaves are strongly absorbed by water molecules. Since most food has a large component of water, microwaves can be used to heat food rapidly. Radio and TV Waves include EHF (Extremely High Frequency), SHF (Super High Frequency), UHF (Ultra High Frequency), VHF (Very High Frequency), HF (High Frequency), MF (Medium Frequency), LF (Low Frequency) and ELF (Extremely Low Frequency). Frequencies λ AM 530 khz 1710 khz 570-186 m FM 88 MHz 108 MHz 3.4 2.8 m TV 54MHz 88 MHz and 108 MHz to 890 MHz 5.6 0.34 m The FM band runs in a gap between channels 6 and 7 of the TV band. This is why you can sometimes pick up channel 6 on your FM radio. Channels 13 and higher transmit in the UHF region. Cellular phones work in the UHF band also. Power waves are the 60-Hz frequency that results from electricity coming from alternating currents used in all wall sockets. The wavelengths here are 5000 km. There is some concern about the effect of low-frequency waves on biological matter; although the current opinion right now is that the typical everyday exposure to these waves is not seriously harmful. The Visible Range The wavelengths of the light that our eyes can see range from 400 nm to 700 nm. Other animals have different ranges. Snakes can detect infrared radiation and insects can detect well into the ultraviolet. Demonstration: Diffraction gratings or prisms can be used to split up the different colors of light from white light. 2002 University of Nebraska. 6

Objects can have color in one of four ways emission absorption reflection transmission Color and Radiation Emission All objects emit radiation due to their temperature, although we generally notice this only when the object is really hot for example, a glowing log, or the filament of a light bulb. If you were able to measure the radiation coming from an object, you would find that the object was emitting radiation over a very broad range and not just over the visible wavelengths. An object that perfectly absorbs or radiates all of the energy incident upon it is called a blackbody. An example of a blackbody might be an oven or a kiln with a very small hole in it that allows you to sense the radiation. If you plotted the intensity of the radiation as a function of wavelength coming from an object at several different temperatures, you would get a plot looking something like Figure 2. As the object gets warmer, the peak wavelength at which it radiates gets lower. By sensing these shifts in the maximum emission wavelength, you can actually tell how hot something is. If you ve ever wondered how Figure 2 : The Wien displacement law you can measure something very hot like molten metal where any thermometer would λ m melt the solution is to use an instrument T = 6000 K called a bolometer, which measures the wavelength and thus the temperature. λ m The wavelength of the maximum intensity λ m obeys a relationship called the Wien displacement law. λ m T = 290. x10 3 m K T = 5000 K Radiation Intensity T = 4000 K For reference, some other temperatures of common objects are: W avelength (nm) Daylight 5500 K cool white fluorescent bulb 4200 K Photoflood 3200-3400 K Warm white fluorescent 3000 K 100 W incandescent 2900 K 40 W incandescent 2650 K 2002 University of Nebraska. 7

Absorption, reflection and transmission Objects also have a color due to absorption, reflection and transmission of light. Most objects can do two or more of these at one time. Absorption occurs because of the molecular form of the material: certain wavelengths of light go into the materials and are trapped by the material. Other wavelengths are reflected. You see only the light that is reflected, so if some light is trapped, you see that the object has a certain color. Filters are examples of materials that transmit light. A red filter will allow red light to pass, but absorbs shorter wavelength light such as blues and greens. A blue filter will allow blue light to pass, but blocks green, red and yellow. You will see some simplified diagrams of light going through filters, like the one shown in Figure 3. While this is a good way to think about the way light behaves with filters, realize that real filters don t quite block light this selectively. Each filter comes with a spectral curve that shows you exactly how that filter absorbs and transmits light. For example, the spectra curve from #861 ( surprise blue ) shows that it passes wavelengths peaked around 450 nm the best. It also passes colors of red starting about 650 nm and continuing into the infrared. The spectral curve tells you not only what colors are passed, but how much of the light is transmitted. Compare this with the spectral curve from filter #877 ( medium blue green ). Here, the main absorption wavelength is shifted to about 500 nm, and you can see that the color is much greener, as you would expect. You can control exactly the effect you want by judicious selection of filters and combinations of filters. This can be important when lighting a house or taking photographs. Color Addition You have probably mixed paint or makeup or ink at some point in your life, and you know that you can make different colors from just a few basic colors. You can get colors by adding different colors together. For example: Red + Green = Yellow (R+G=Y) Blue + Red = Magneta (B+R=M) Blue + Green = Cyan (B+G=C) Blue + Green + Red = White (B+G+R=W) Figure 3: Idealized picture of light passing through a filter Red, blue and green are thus know as the additive primary colors. Any color can be made by adding these three together. If you look at your television set, you will find that there are triads of very small dots that emit blue, green and red. By exciting different combinations within each triad, any color can be formed. Color Subtraction Let s say that I want to use filters to mix colors. We call this color subtraction, because the filters remove color. For example, if you look at the spectral chart for a purple filter (#841), you see that the filter blocks green very well, allowing the purple to be formed by the blue and red wavelengths that are allowed to pass. I want to get cyan, so I m going to mix green and blue A white light through the green filter allows only green to pass. If I then put a blue filter after the green filter, I get nothing because the blue filter blocks the green light and there isn t any blue to let pass. This situation in shown in Figure 4. 2002 University of Nebraska. 8

Figure 4: Colors can t be mixed directly by putting light through filters In order to get colors using subtraction of light, we use different colors. Looking back at the color addition, we found Red + Green Blue + Red Blue + Green Yellow Magenta Cyan We define a complementary color to be two colors that you add together to get white. R+G+B = W (R+G)+B = W Y + B = W Yellow and blue are thus complementary to each other. The other complementary colors are green and magenta and red and cyan. If I want to form red, I can start by running white light through a yellow filter, which blocks its complementary color (blue). The rest of the light is then sent through a magenta filter, which blocks its complementary color (green). This process is known as color subtraction, because you are removing the colors you don t want. Figure 5: Using subtractive color mixing to produce red light. 2002 University of Nebraska. 9

How Does the Eye See Color? Color is very subjective. Some of the mechanics are understood, but there are many experiments that show that people don t always see things the same way. We ll talk about the eye in detail later, but you probably already know that the eye has a retina that is lined with two different types of cells: cone cells and rod cells. Both types of cells receive information about what you see and transmit electrical signals to the brain. Cone cells can distinguish between different frequencies of light provided the light is sufficiently intense. Your brain interprets these signals as color. Rod cells are sensitive to light and can distinguish light from dark in low-light situations. One theory is that there are three different types of cone cells, one of which responds to red, one to green and one to blue. Color blindness is then explained by someone missing the appropriate number of cones for one or two colors. 2002 University of Nebraska. 10

VI. PROCEDURE A. Set-Up. The Singerman color apparatus Parts. The Singerman color apparatus is a large black box with a ground-glass screen. There are six filters: 1 red 821 2 green 871 3 blue 856 4 cyan 877 5 magneta 838 6 yellow 809 The filters slide into the slots on the sides (two on one side, one on the other) of the color box. Filters are inserted so that the filter is the farthest away from the light bulbs as possible. If you aren t careful, the filter gel will touch the bulb and will melt, creating a mess. You MUST align the Singerman color apparatus before starting. The demos will not work if you don t! To align the color box, first remove the back panel. Make sure each bulb is in the center of the 2 aperture in front of it. Figure 5: The pattern that results from correct alignment of the Singerman color box. Place the red, green and blue filters in the slots so that they produce the pattern shown in Figure 5 when the light bulbs are turned on. Make sure that the colors are arranged properly, or they won t match the slides. Be careful not to melt the filters on the hot bulbs. If you don t have the overlaps as shown, or if the circles are cut off, you may need to realign the bulbs. B. Execution 1. Show the students the diagram of the electromagnetic spectrum (found in the introduction and also in the color powerpoint). Point out the light we can see (visible light), and describe the other categories, especially the ones the participants are likely to be familiar with (microwaves, radio waves, etc). 2002 University of Nebraska. 11

Explain that the difference between the different kinds of light is that they have different wavelengths. The wavelength is length of one complete wave. 2. Hopefully after seeing the visible light in the electromagnetic spectrum, a participant will ask why white light is not included. If no one asks this question, you can make a smooth transition by posing the question to the participants. 3. Take out the prism, hold it in front of a light source and show the participants that it breaks the white light up into colors. Emphasize that white light is all the colored light put together. 4. Now that the students understand that all the colors together make white, show them that two different colors put together can make another color using the Singerman color box. Put together various combinations of colors and see which colors they make. The color powerpoint has slides of the color combinations which may be used along with the color box. Tell the participants that red, green, and blue are the additive primary colors because any color can be made by combining those three colors. The shades of the colors can be varied by changing the brightness of the light. Also, when all three primary colors are put together they make white. Remember, white light is made of all the colors. 5. To demonstrate subtractive color mixing, take out the subtractive color discs (3 colored plastic circles). Overlap the magenta color filter and the yellow color filter and hold them up to the light or place them on an overhead projector. The part where the filters overlap should appear red, as demonstrated in figure 5 of the introduction. For more diagrams like figure 5, see the color powerpoint. Try different combinations. Tell the participants that the color seen is the only color not blocked/filtered by the color filters. The subtractive primary colors are yellow, magenta, and cyan. 6. Mixing dyes and paints is actually subtractive color mixing. For example, red dye or red paint absorbs all the colors except for red. The color paddles mix like paints. For example, if you mix blue paint with yellow paint, you will get a green paint; if you overlap the blue paddle with the yellow paddle and hold up to the light, they will appear green. Overlap two color paddles and see the color change. 2002 University of Nebraska. 12

VII. FREQUENTLY ASKED QUESTIONS VIII. TROUBLE SHOOTING IX. HANDOUT MASTERS X. REFERENCES: Color powerpoint Color script (see page 14) Color filters can be purchased at www.arborsci.com (item number 33-0175) 2002 University of Nebraska. 13

1 should dress rather loudly pauses, waits, looks expectantly at. Hi. We re from ScienceWorks, an outreach group from the University of Nebraska. I m and this is. (Say a few words about your department, what you do, etc.), it s your line I though we talked about this last time. How am I supposed to work with you wearing that that outfit. 2002 University of Nebraska. 14 What s wrong with my outfit? Nothing. If you re colorblind. Last week it was magenta and green, the week before red and cyan. What s wrong with magenta and green? They re complementary colors. What do you mean they re complementary? Not to each other, they aren t. looks a little confused I m confused. I ve been studying colors all this time so that I can do the color show. I m sure the script said that magenta and green were complementary. No, no, no. When we talk about colors being complementary, we don t mean that they go with each other! Now I m really confused. 2 Well, maybe while we explain color to our audience, we can clear up your confusion. Lights go out I ve been studying. Ask me anything. OK. Let s start by turning out all the lights.

Hey I can t see anything. That s right. And when it s very dark out, what colors do you see? Well, you don t see any colors. Everything looks mostly grey Lights on Ah ha. So in order to see colors, you need to have light. OK. I ll buy that. But light doesn t have color itself, does it? Some light does But not all light does? 3 In order to understand color, we have to understand light. Let s look at something called the electromagnetic spectrum. The what? 2002 University of Nebraska. 15 The electromagnetic spectrum. It s a big name, but then again, it s a big spectrum. OK. Now I have absolutely NO idea what you re talking about. Not really. Bear with me. Light is a wave. You mean like a wave on the ocean? 4 Sort of like that. The waves that make up light also go up and down. They have what we call a wavelength. It looks to me as though the wave is repeating itself. If you cut out this part (indicate one wavelength), you could just copy it lots more times and you d have the same thing you have there. 5 That s right. The distance over which the wave repeats itself is called the wavelength. I see

Waves come in all different wavelengths, from very, very small, to very very large. Light is just a small part of the electromagnetic spectrum. The electromagnetic spectrum is the 6 range of all the wavelengths we know about. This is what it looks like: Wow! That s a lot of waves Let s go through them and you ll see that it s not so complicated. OK. The electromagnetic waves that have very small wavelengths are waves like gamma rays and x-rays. Microwaves and the types of waves that bring in radio signals are waves with very long wavelengths. The waves look, well, sort of grey, don t they? I thought we were talking about color. The thing about all of the waves I mentioned is that they are waves that we can t see. Our eyes only see a very small portion of all the waves in the electromagnetic spectrum. So those must be the ones in the middle. 7 That s right. Here, let s expand that region a little bit so we can see it better. Hey! It s a rainbow! 2002 University of Nebraska. 16

That s right. We can see waves when their wavelengths are within a certain range. Depending on their exact wavelength, they appear to us as different colors. Use this for older audiences only What s the 400 nm and the 700 nm mean? nm is the abbreviation for nanometer, which is one billionth of a meter. That s awfully small. Right. About one million nanometers can fit across a period in a book. That s a very small unit of length! So you re saying that we can see light waves when they re between 400 nanometers and 700 nanometers. 400 nanometers is a smaller wavelength and 700 nanometers is a larger wavelength. That s right That s not very much of range, is it? It s not very big, but it is amazing that we can distinguish between all of these colors. Wait a minute! You re missing a color - where s white? What do you mean where s white light? Look at your diagram. I see red, orange, yellow, green, blue, indigo and violet. Where s white? It s there. I don t see it. White light is made up of all the colors together 2002 University of Nebraska. 17

8 This slide is blank to provide a backgroudn 2002 University of Nebraska. 18 No way! If you look at the light coming out of the projector, for example, it s white. I don t see any colors. pulls out a prism If I take this prism and place it in the path of the light coming from the projector, what do you see? Point out the colors Hey! There s all of the colors again red, orange, yellow, green, blue, indigo and violet. And they all come from the white light that s coming from the projector. That means that white light is the combination of all different colors of light at the same time. 9 That s right. And combining colored lights to make other colors is a principle called color mixing Pulls out Singerman color box I know about color mixing. After all, I am quite artistic. Yes, I ve heard. I know! I have really good demonstration for color mixing. Here it is! Hmmm. A black box. It s not just a black box. (Turn on one light) Oh! It lights up. Yep. And we have these neat filters that allow us to color the light that you see. But you only have three of them red, blue and green. What about the other colors in the rainbow? Color mixing! (to audience) How many of you have every mixed paints to get a different color? (To ) See they know what I m going to do.

Well, I don t. What are you going to do? 10 Turn on two lights Let s start with mixing red and blue. What do you think we re going to get? To make it easier, I ve narrowed it down a bit. Here are your choices: brown, yellow, magneta and cyan. Wait a minute! What is magenta? Magenta is a fancy name for purple. When artists and scientists who study color talk about this particular shade, they always call it magenta, so I thought we should too. And cyan? Same thing. Cyan is a particular shade of blue-green. So now that we understand the choices, let s take a show of hands. Go through choices one at a time and see how many people think each color will result. Many should get this one right. 11 turns the intensity on the red and blue lights up to full Well, don t keep us all in suspense. Let s do the experiment. Ta dah! Magenta! OK, but what happens if you mix red and green? Let s find out by doing the experiment. But first, let s ask the audience what they think. 2002 University of Nebraska. 19

12 OK. If you mix red and green, do you get: brown, yellow, magenta, or cyan? (Ask audience how many think each one and take a show of hands) Personally, red and green are both pretty dark, so I think you re going to get brown. 13 Let s do the experiment and see. turns on the red and green bulbs Hey, look at that. You get yellow. Yellow? How do you get yellow from red and green? I m not sure, actually. Maybe we should continue and see what else we get. 14 OK. All that s left to mix would be blue and green. (To audience) Your choices this time are, surprisingly, brown, yellow, magenta and cyan. (Take show of hands) What do you think this time,? I m guessing cyan myself, because you said earlier that it was a bluegreen color and if you mix blue and green, you ought to get something blue-green. 15 OK. Here goes. You re right! Cyan is the correct answer. But that s only three more colors. Now you have six. You said that all colors could be made from just red, blue and green. I only saw three more colors. 2002 University of Nebraska. 20

Be patient,. You ll notice that I can vary the intensity of each of the colors. Using the example of blue and green, if I slowly decrease the amount of green, you can see that what was cyan changes color. If I have a lot of green, I get cyan. If I have a little green, I have a light blue. If I have no green, I just get the darker blue. Turn green all the way down. I see. So if I turn on the red gradually, I see a lighter purple and finally the magenta color. Then if I slowly turn the intensity of the blue light down, I get a color that is more pink than purple. With no blue, I just get red. I wonder what happens if we turn them all on at the same time. What should we see? I don t know. Maybe someone in the audience knows. (Take audience guesses.) Let s go ahead and try it. Turn on all three lights at the same time. So now we see the cyan, the magenta and the yellow. And in the center is 16 White! So you re right. Not only can you break up white light to make different colors, if you mix red, blue and green together, you also get white. 2002 University of Nebraska. 21

17 So let s summarize. If you mix together red and blue, you get magenta. If you mix red and green, you get yellow, and if you mix blue and green, you get cyan. 18 Because you can get all of the colors from red, blue and green, these three colors are called the additive primary colors. In fact, your television set works using color addition. I thought it needed electricity It does. But the colors that are formed are formed from dots of just three colors. Red blue and green? 2002 University of Nebraska. 22 That s right. The television screen is covered with little tiny dots some use very thin lines that glow in red, green or blue. They are arranged in groups of three across your screen. 19 Sort of like this? Yes. By lighting up each of the dots in the proper intensity, any color light can be produced on the TV screen. That explains why the three additive primary colors are so important. But did you know that the three colors that the primary additive colors make are also an important group of colors? They are? 20 Sure. Red, blue and green in various combinations make cyan, magenta and yellow. OK.

21 Cyan, magenta and yellow are called the subtractive primary colors. 22 Hold up filters to show that they pass the different light colors. Subtractive? In the color mixing that you showed, we got different colors by adding together two or three colors. In subtractive color mixing, we use filters to remove certain colors. We used filters here, though, to make red, blue and green light. That s right. A red filter is designed to block all colors of light except red. That s why light that passes through the filter is red. The screen allows us to combine the different colors of light again. We know that white light is just red light, plus green light, plus blue light, 23 so we could actually draw this like this (picture) I m not clear on what you re saying. Can you give me an example? Let s think about adding red and green. What do we get? Yellow. 24 Right. Now let s take white light and put it through a yellow filter. What do we get? 25 Yellow light. Which is Yellow light. Use prism to break up yellow light. No, I mean what is yellow light made of? Let s use a prism to find out what colors are in yellow light. It looks like mostly red and green light. 2002 University of Nebraska. 23

26 That s right. So the yellow filter actually passes red and green light. But what about the blue light? What about the blue light? We saw that white light is what you get if you add red, green and blue, right? Yes. 27 So I can schematically illustrate the white light by red, green and blue light. Yes. And what you re saying is that the yellow filter lets through the red light and the green light, which we see as yellow. Right. But that means that the yellow filter must block the blue light 28 That s right. Yellow filters block blue light and let through red and green light. We say that blue and yellow are complimentary colors. I ve always thought they looked nice together 29 No, no, no. Complimentary is the term that we use to denote two colors that you add together to get white. Huh? 30 You believe that yellow light and blue light makes white light, right? I m not sure 31 Yellow light is red and green light, right? So when we add yellow and blue, we re actually adding red and green and blue, which is 2002 University of Nebraska. 24

White. It s very nice to see up on the screen, but does it actually work? takes out the red and green filters, turning down the intensity on each, and puts a yellow filter in. Let s see. I should be able to replace the red and green filters with a yellow filter, right? Yes. And what should I get when I turn on the lights? You should get white, according to what you ve told me. turns on the blue and yellow filters See, you do get white. So we say that blue and yellow are complementary because they add up to make white light. turns down the intensities on each light and removes the blue and yellow filters and inserts cyan and red. So I should be able to take cyan and red and add them together and get white, too, right? Let s find out. It works! So cyan and red are complementary colors. What does this tell you about magenta and green? The fact that magenta and green are complementary just means that they add up to give white light. Not that they look good together. 32 which I think you ve given us a very good demonstration of today. So the pairs of complementary colors are: blue and yellow, green and magenta, and red and cyan So, my sweater is yellow that means that they used red dye and green dye, right? 2002 University of Nebraska. 25

33 ADD SLIDE - ;subtractive No. What we just showed was how light can be mixed to produce different colors. If you re talking about mixing paints or dyes, there are different rules. We call this type of color mixing subtractive color mixing. What do you mean subtractive? How do you take away color? If you look at this red dye in the water, what do you see? Take suggestions from audience This is a trick question, right? I see red. That s right but why do you see red. Well, when we saw red light from a filter, we saw it because the filter blocked blue and green only the red light got through. 34 That s how red light works. When you re using paints or dyes, you see red because the dye actually absorbs all the colors except red. I see so it s slightly different than when we were adding lights. Let s see if we can figure out the rules for subtractive color mixing. What do you think would happen if we mixed red dye and green dye? does the experiment, showing that the intersection is indeed green Well, last time, we saw that when we mixed red light and green light, we got yellow, so I m guessing yellow. Mix red and green dye you will get brown That s one possibility. Does anyone in the audience have another idea? (Take ideas) Wait a minute. Something s wrong here. This looks like brown 2002 University of Nebraska. 26

But think about what I said about how dyes produce color by absorbing other colors. 35 I see! So red absorbs all colors except red. And green absorbs all colors except green. So together, they absorb all colors. Can we do subtractive color mixing? 36 If you have the right colored filters, you can. Just like red, green and blue are the additive primary colors, there are also subtractive primary colors cyan, magenta and yellow Hey didn t we have cyan, magenta and yellow before? Yes, we did. I have some pieces of colored plastic and they behave very much like paint or dye if you overlap them. So if I combine the cyan and magenta pieces of plastic, what do you expect to see? Brown! Why brown? We mixed red and green and got puts together cyan and magenta filters, producing blue brown (Exasperated, to audience) What do you expect to see? Wait! It s blue. Do I understand this? Let s see. Cyan removes red, and magenta removes green so the only thing that is left is going to be blue. I guess I do understand this. 2002 University of Nebraska. 27

Let s see if you do understand this. What happens if we mix magenta and yellow? (Take audience guesses) Well, magenta removes green and yellow removes blue, so I guess Do demo get red. we should get red. What happens if we put them all together? Let s find out. So we find that magenta and yellow produces red, cyan and yellow produce green and magenta and cyan produce blue. And what about the combination of all of them? 37 Since each of the filters removes one of the additive primary colors, there s nothing left, and we get black. 38 So we hope we ve taught you something about light and color today. We introduced you to the electromagnetic spectrum and showed you that the light we can see is only a small fraction of the spectrum. 39 We showed you that red, green and blue are the additive primary colors and that they can be combined to form any other colors, including white, which the sum of all three colors. 40 We also showed you that you can also mix colors by color subtraction, where the primary colors are cyan, yellow and magenta. If you subtract all the light, you get black. 41 We d like to thank you for your attention and answer any questions you might have. 2002 University of Nebraska. 28

COLOR SHOW NOTES Turn down the intensities on the bulbs when changing filters and when you re not using the apparatus. It gets hot very quickly. You may have to adjust the intensities slightly to get the right colors due to differences in the transmission of the different filters. 2002 University of Nebraska. 29