Q: What s Going on When You Mix Colors?

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1 Background boosters for elementary teachers Q: What s Going on When You Mix Colors? By Bill Robertson A: Artists of all kinds mix different colors together, whether they are drawing and painting or lighting a stage production. It would behoove artists, then, to understand all sorts of things about color mixing. And it would behoove teachers to understand color mixing so that students can understand it. So, what s to learn? and blue make purple, blue and yellow make green, and mixing all colors together makes mud. Or do they? Sometimes yes, and sometimes no. To explore color mixing, it will help a lot if you have access to colored filters red, blue, yellow, orange, and green will do nicely. You can order sheets of decent filters (which you can cut into small sections) from science supply outlets. I use rainbowsymphony.com, but there are others. If you know a photographer, then you might be able to borrow different-color filters from that person. You can use different-color plastic wrap in a pinch, but that won t lead to great results. Before you start, it will help if you realize that white light is composed of all the colors of the rainbow. I ll address how we know that later on. For now, let s just assume that it s true. Also before we begin, I m going to warn you that I ll be making a few simplified, not-always-true statements throughout the column. The plan is to make clear what my We have every visible wavelength in stock. What color would you like me to filter out? tiny lies are by the time I finish. This being an election year, that puts me miles ahead of lots of what you ll hear on TV. So, let s suppose you have access to at least a few different-color filters. Look through a blue filter, say, at a white light. What color do you see? Yep.. If you were to shine a flashlight through a blue filter onto a white sheet of paper, you d see blue light. That s because blue filters block out all colors of light except blue light. See Figure 1. Similarly, a red filter blocks out all colors except red light, a green filter blocks out all colors except green light, etc. (This is the start of my tiny lies. Accept them for now and I ll explain later.) Now, answer a question. What color of light reaches your eye when you re looking at a red shirt? If you answered red light, good for you. Similarly, a blue shirt is sending blue light your way. Time for your first surprise. Look at a red shirt while holding a blue filter up to your eye. What color is the shirt? Look at a blue shirt while holding a red filter up to your eye. What color is the shirt? If you have reasonably good filters, your answers to the last two questions should be black. But why is that? Aren t red and blue supposed to combine to make purple? To understand what s going on, I m going to introduce a graph that shows the different colors of the rainbow and which of those colors an object produces or a filter blocks out. For example, Figure 2 shows the colors produced by a red shirt. Figure 3 shows this same kind of graph but labels the colors that a blue filter allows to pass through it. Figure 4, page 64, shows these last two figures combined, illustrating the light coming toward your eye and the light the blue filter allows to pass through it. If you re looking at a red shirt with a blue filter in front of your eye, how much light should reach your eye? If the blue filter only allows light BRIAN DISKIN 62

2 FIGURE 1. blue filter white paper that s inside the shaded blue region to pass, and the red shirt produces only the light that s inside the shaded red region, then no light at all should reach your eye. No light at all means black. Therefore, when you look at a red shirt while holding a blue filter up to your eye, the shirt should look black. Also, a blue shirt viewed through a red filter will look black. By that same reasoning, if you put a red and a blue filter on top of each other and look at a white light, it should look black. It doesn t quite, though, because most red filters, like all filters except the most expensive ones, don t just let one color pass through. Most red filters allow a bit of purple light to pass through, so the white light you re looking at through the filters will likely have a purplish tinge to it. If you re keeping score, I just corrected a tiny lie that I told you. Next, get a blue filter and a yellow filter and place them on top of each other. Look through them at a white light. If you see green, then you re not color-blind. I told you that filters don t just let a single color pass through them. and yellow filters both allow a fair amount of green light to pass through them. What you see, then, when you look through both filters at once, is the region of overlap. See Figure 5, page 64. What we ve been dealing with so far is known as color subtraction, because filters subtract a bunch of colors from white light. Mixing oil and other opaque paints is also a process of color subtraction. Yes, you add the paints to one another, but because oil paints absorb white light and then reradiate only certain colors, it s a color-subtraction process. white light FIGURE 2. Light that reaches your eye from a red shirt FIGURE 3. blue light Light that the blue filter allows to pass February

3 When you mix together lots and lots of oil paints, you get a muddy concoction that approximates black (all of the colors removed). As any artist knows, though, mixing paints isn t quite as simple as basic color subtraction, especially when dealing with watercolors or paints that have been thinned significantly. Time to move on to something that s not color subtraction. That would be color addition. There are several ways to experience color addition, one of which is to use the simu- FIGURE 4. Light that reaches your eye from a red shirt FIGURE 5. Light that yellow filter allows to pass region of overlap lation found in this column s Internet Resource list. That simulation allows you to add together different intensities of red, blue, and green light. If you have three flashlights, differentcolor filters to put over them, and a friend, you can experience color addition in real life by setting up a situation such as Figure 6. What you re doing is shining different colors of light onto the same spot on a white sheet of paper. With either the flashlights or the simulation, add together different Light that the blue filter allows to pass Light that blue filter allows to pass colors and different amounts of those colors. You should notice that combining colors in this way doesn t give you the same results as color subtraction. For example, adding red and blue certainly doesn t give you black. If you add lots of different colors onto the same spot (maximum intensity of all three colors in the simulation), you end up with white light, or a close approximation to white light. How come? Well, remember that white light is composed of all the colors of the rainbow. Therefore, if you add together all, or most of, those colors, you re going to get white light. Figure 7 shows why this happens. Adding more and more colors gives you more and more of the spectrum of color, and that gets you closer and closer to white light. The most obvious artistic application of color addition is stage lighting. Stages are lit using many separate spotlights, each of which can have filters that result in the production of different colors of light. The separate spotlights add together on stage. A less obvious artistic application is the Impressionistic painting method known as pointillism, in which the painter uses thousands of tiny dots for the painting. You can see the dots when up close, but when far away, your eye combines the different colors to create what appears to be smoothly transitioning colors. Though arguably much less artistic, your computer screen and your television set use a form of pointillism. The screen is covered with red, blue, and green phosphors (things that glow) that generate all the necessary colors for a picture. Your eyes and your brain combine the tiny, colored dots. 64

4 My compadre in crime, artist Brian Diskin, will be the first to tell you that using color addition and subtraction in art is, well, artistic. Though science is at the heart of color mixing, there is more art than science in its application. There s no such thing as a single red, blue, or green color; and those labels on your 64-color Crayola box don t begin to outline the many varieties of even a single color. Also, the paints alone do not tell the story. The color and texture of the canvas affect color-mixing results, as does the density of the paints themselves. When you use watercolors or significantly thinned oils or acrylics, a large amount of white light, reflected from the canvas, reaches your eye. That means the simple color subtraction doesn t necessarily apply. I ll end this column with a shameless self-promotion. The basic ideas I ve outlined here form the basis of many applications, from 3-D comic books to how the old Password board game worked. And I haven t addressed human color perception, which could fill several columns alone. Where might you discover all that neat stuff? Why, in the book, Stop Faking It! Finally Understanding Science So You Can Teach It: Light. I hear the book is pretty good. I know the author. Bill Robertson (wrobert9@ix.net com.com) is the author of the NSTA Press book series, Stop Faking It! Finally Understanding Science So You Can Teach It. Internet Resource Color Addition Simulation color-vision/latest/color-vision_en.html FIGURE 6. FIGURE 7. In this area, you are adding separate colors. Add more flashlights with filters to add more colors. blue light yellow light Different colors added together give you more and more of the complete spectrum, which is white light February

5 Exercises to challenge content knowledge This Month s Task: Measuring Up Get a meter or yardstick. Rest the two ends on your index fingers. Begin sliding your index fingers toward the middle of the stick. As long as you don t grab the meterstick, your fingers will slide alternately toward the center, and your fingers will always meet exactly in the center of the stick. Why? Find out next month, or now in the digital issue of Science and Children. Mirror, Mirror Last month, I challenged you to draw various shapes, first looking directly at the sheet of paper and then while looking at the paper via a mirror. The second task was no doubt extremely frustrating! The pen doesn t seem to go where you want it to go, and you can even get paralyzed trying to make the pen go in the proper direc- tion to draw the shape. To understand why the mirror task is difficult, all you have to do is watch a baby for a while (it helps if the baby is awake). Babies tend to be clumsy. They reach for things and have trouble grabbing them. They sometimes have difficulty focusing, and they certainly aren t very good at getting a spoon full of food into their mouths. What babies are doing is coordinating the input to their brains (touch and sight, for example) with the messages their brains are sending to their body parts. If the sense to brain to movement connection were easy, then babies would just start walking without any trouble. By the time you re an adult, you ve mastered those connections. You desire to move a pen a certain way on a piece of paper, and you just do it. By looking in a mirror, however, you are messing with your brain. Your eyes are telling you to move your fingers one direction, but that movement doesn t get you the desired result. The pen you re watching through the mirror doesn t go in the direction you expect it to. Can you train yourself to draw shapes while looking in the mirror? Absolutely. It just takes practice. There s a classic scientific study that illustrates this process. Subjects in a study were given goggles that turned everything they saw upside down. They were forced to wear the goggles 24 hours a day. At first, the subjects were nauseous and disoriented, but after a few days, something remarkable happened. With the goggles on, the subjects began to see the world right side up again. Their brains adjusted to the disconnect between their movement and what they were seeing. Just as interesting, the subjects went through a period of seeing the world upside down after they removed the goggles! Eventually, things returned to normal. The moral of the story is that human brains are quite interesting and, at times, quite funny. 66