Color Part I Name Color is one of the most important pieces of information scientists have used for all time. In space it is one of only two pieces of information we can collect without sending probes out (for millions of dollars). Color is how we determined that the Sun is a typical star and what the Moon was made of. It's how we know where the Earth came from and how long the Sun has to live. It tells us more about the universe than anything we could gather even if we could send out hundreds of spacecraft. (The two items we can determine: a. How bright is the light is. b. What color the light is.) First, what is color? Color is caused by different frequencies of light. (Huh? Frequency? - Frequency is how often. For instance the frequency of you doing your homework is daily. Wait, make that the frequency of you doing your homework should be daily. The frequency of you eating is about 8 hours. The frequency of you changing classes is every 48 minutes.) When we talk about light frequency is the measure of how often the light wave (sort of like an ocean wave) goes up and down. For visible light that frequency is in around 1,000,000,000,000,000 waves every second! Very slight changes in that frequency are detected in the cones of our eyes to be different colors. Unfortunately our eyes are very poor detectors of electromagnetic waves such as light. Your eyes detect a very small portion of those waves. There are many more waves that we can't detect with our eyes. (See electromagnetic spectrum of your text.) We've defined many colors by the frequency of the light that we receive. We could do calculations on these, but it isn't really necessary for us to understand color or how to use it. Instead we will look into how we perceive light and how light works. The first thing we have to do is revise the color wheel you learned in grade school. It worked for what you were working with, but it s now time to get a little more sophisticated. The primary colors of light are red, green and blue. Eeek! Heresy you say? Think about it. What are the three colors on a TV screen or computer monitor? Yep, red, green, and blue. What is the color scheme in a camcorder or electronic camera? Yep, RGB. How about the lights on the stage? The strip lights the theater uses have red, green and blue lights in them. So what happened to the red, blue and yellow of grade school? Those are pigments and they work a little differently. We'll discuss them later. For light, the primary colors are red (R), green (G), and blue (B). The secondary colors are combinations of equal amounts of the primary light colors. When you combine R and B you get a purplish-red color called magenta (M). When you combine G and B you get greenish blue color called cyan (C). When you combine R and G you get a reddish green color called yellow (Y). Yes, yellow! It doesn't make much sense from your paint mixing days, but it does for your computer monitor days. You can combine all three colors to get white light, also. Your eyes have three different (sometimes four) different types of color detectors. You guessed it, RGB. You have cones (the things in your retina that detect color) and most people have three different types. Some people have two different types of red, so they are able to detect red in two different peaks. When you combine detection by two different cones, you get the other colors. The rest of the colors you see on your TV or on your computer are made up of those three colors, or lack thereof. Check out the color settings on your computer. Most have the three colors and you can vary the intensity of each to get whatever color you need.
The definition of complementary colors is different for a physicist than they are for an artist. (But the colors are similar and you find them the same way on a color wheel.) For a physicist, complementary colors are two colors that when combined in equal intensities make white light. R and C are complementary. B and Y, G and M are also complementary. You can find them because they are on opposite sides of the color wheel. (Just like in art class!) They are important to physicists because they help define what light we are missing and what is there. Complementary colors are important to artists because they are two contrasting colors that go together. Classic school colors are often complementary colors. When adding colors of light together people often refer to it as color by addition. When you put more colors together you get more light. When you add pigments you get color by subtraction. More on that later. Today we are going to do a lab to help you remember the color wheel for light.
Light (color by addition) Name Fill in the seven sectors shown. Use the three primary light colors in the large outside circles. (RGB system) (Remember, paints and crayons aren t ideal pigments.) Y=Yellow C=Cyan G=Green R=Red B=Blue M=Magenta W=White K=Black Each circle represents a spotlight of the color indicated. Fill in the missing light colors.
Color Part II (Pigments) Name Now we get into the second half of the color analysis. This is often referred to as color by subtraction or pigments. Pigments absorb one or more colors of light and reflect the rest. Ideal pigments are familiar to anyone who does any work with the four-color printing process, color copiers, and color inkjet printers. The ideal pigments are ones that absorb one color and reflect the other two. The ideal pigments are cyan, yellow and magenta. (Dang, those elementary school teachers were still lying to us!) The abbreviation for this printing process is the CYMK (cyan, yellow, magenta and black) system. Check the color ink cartridge in your ink jet printer or the toner cartridges in your color copier. You can get most colors with this process, although you can't get an unlimited number of colors because you can't raise the intensity of the colors beyond the four inks you put into the cartridges. Here's how each color works. To see a color from a pigment or through a filter the pigment has to reflect the correct color of light (or transmit for a filter). For instance to reflect magenta you need equal intensities of red and blue light, but no green light. Therefore magenta pigment must absorb green light and reflect red and blue. Cyan has to absorb red and reflect blue and green. Yellow has to absorb blue and reflect red and green. (Note each ideal pigment absorbs its complementary color.) If you mix pigments, each pigment absorbs some color or colors of light so each pigment keeps absorbing more and more light. (color by subtraction) If you mix cyan and magenta you get blue. (Cyan absorbs red light and magenta absorbs green. The only light left to reflect is blue.) Magenta and yellow makes red (magenta absorbs green and yellow absorbs blue) while yellow and cyan make green (yellow absorbs blue and cyan absorbs red). Now you have to make a separate color wheel for pigments. It's the more common one, but it's not exactly like the one you have in art class, but we'll explain that later. You can still find complementary colors (two colors that combine to make white light) on opposite sides of the color wheel and they still go together. Pigments and filters work the same way, except filters are made to pass light through. Lighting managers for the stage use these to their advantage all the time. Remember, if a filter doesn't allow a light to get through, that light can't reflect off the items on the stage. This can be used to dramatic effect. I saw a scene once where all the actors were dressed in yellow and they went through the scene in normal (amber) lighting. At the end of the scene a terrible accident was supposed to have happened and people died. The director changed instantly from amber to blue lights (a blue filter over the light allowing only blue light through, absorbing green and red light) - and All the actors' costumes turned from bright yellow to black - blue has no red or green light to reflect off the yellow pigments in the costumes. What about those Prang watercolors I used to mix and or the acrylics I use in the art department? Well, they aren't ideal pigments by any stretch of the imagination. In fact the watercolors you used in grade school were mostly the cheapest pigments possible to slap into a little case. The yellow wasn't a perfect yellow. It reflected lots of red and green, but it also reflected some orange and some yellow light (Absorbing blue and violet). The blue reflected lots of green and violet as well as blue. (Absorbing red, orange and yellow.) When you mixed the blue and yellow watercolors together, all the colors were absorbed except for green.
Even with more ideal acrylics or oil paints, you have to be very careful of how you mix the colors. If you mix a very bright red with a very bright yellow you get a brownish orange - not what you expected. Artists have to do much trial and error to get just the right colors for a painting. They are truly scientists experimenting, often writing down what worked and what didn't (recording in a notes). If you want to see how pigments are used, check out the Sunday comics under a converging lens. You can see that they are printed from multiple dots overlapping to form different colors. This is very similar to the method they use to print color pictures in a textbook, except the textbook's dots are much, much smaller. Some of the most valuable Pokeman cards are misprints where the printers messed up the print and the four colors (CYMK) don't overlap perfectly and the card looks like it is out of focus. We even had to return some of our textbooks to the printer because of the same problem. (Strange that misprint Pokeman Cards, Baseball Cards and Postage Stamps are rare and valuable. Misprint textbooks are worth much less and usually destroyed.) Spectrum There are three basic ways to get a spectrum our visible light. (although there are many variations) First there are prisms. They take advantage of the fact different colors (frequencies) of light travel at different speeds through a medium. Red changes speed the least, so it bends the least. Violet changes speed the most and therefore bends the most. This creates some separation when white light goes through a prism. Note: this can create a problem for lens telescopes. Since lenses are really a bunch of prisms stacked on top of each other, the lenses also separate the light into the spectrum. This is known as chromatic (color) aberration. This is a result of all colors not focusing to the same point. Look through a hand lens. At the edges you will notice that things change color slightly. This is the color aberration. Even camera lenses have this problem. (Mirror telescopes don't have this problem!) High quality telescope and camera lenses actually use two lenses of different glass (different index of refraction) glued together to adjust for this. It makes these lenses more expensive, but they work better. Diffraction gratings also separate the colors into the ROYGBV. You've seen this when looking at the etched side of a Compact Disc. You see a spectrum because of the waves interacting with each other. We will be looking into that phenomenon later, but you will be able to use some diffraction gratings later to look at some elements and see how we use them to study color in even more detail. Rainbows are my favorite way to make a spectrum. To study them in more detail, first log on the Internet and check out the Earth and Sky website (our website has links to the shows you need.) http://original.oprfhs.org/division/science/faculty/kmccarron/ils-1/rainbows.html
Primary rainbow Secondary rainbow Once again, you notice that there is some refraction as well as some reflection to make a rainbow. Rainbows also take advantage of the different indices of refraction for different frequencies of light. As you look at a rainbow, the Sun must be at your back and the rain drops must be ahead of you. There's always a double rainbow, most often the second bow is too dim to see.
Homework Questions 1. Do rainbows show reflection or refraction? Explain. 2. What conditions do you need to have a rainbow in the sky? 3. What is the difference between two different colors of light? 4. How do we detect color? 5. An actor stands where two spotlights cross. One is red and the other green. What color light is the actor standing in? What color are the shadows? 6. Why is the sky blue? 7. Why is the sunset red? 8. What is the absence of all color? 9. A ship painted red, white and green sinks to the bottom of the ocean, where there is only blue and green light (the red is absorbed above). What colors will the ship appear to divers using the natural light of the ocean? 10. What are the ideal pigments? Where might you find them? 11. What are the primary colors of light? Where might you find them?
Pigments (color by subtraction) Name Fill in the seven sectors shown. Use the three primary pigment colors in the large outside circles. (CYMK system) (Remember, paints and crayons aren t ideal pigments.) Y=Yellow C=Cyan G=Green R=Red B=Blue M=Magenta W=White K=Black What is the difference between color by addition and color by subtraction? Using the diagrams below determine what color light will get through the filters.
Fill in the missing pigments using the CYMK system.