Light and Shadows- in Color
[Shopping List: Red, green and blue party lights]
- Hold your hand up near the wall where the colored lights are pointed to cast a shadow. What colors do you see? Where did all those extra colors come from?
What's Happening: First suppose we have just one colored light bulb, say the red one, shining on the wall. A shadow is formed where the light is blocked by our hand- no light reaches areas within the shadow, so they are dark or black, while the rest of the wall is bright red because light is being reflected from those areas into our eyes. Now suppose we turn a second light bulb on, a green one, and place it in a slightly different location. The shadow of our hand cast by the green light will be in a different area of the wall, although most of it will probably overlap the shadow from the red light. Now most of the wall where there are no shadows is yellow, because yellow is what we see when the red and green light being reflected into our eyes is combined. But in some parts of the shadow that was cast by the red light (i.e. there is no red light reflecting from these areas) there is now only green light reflecting off of the wall into our eyes, so these areas appear bright green now instead of black. Similarly, in some parts of the shadow cast by the green light there is now only light from the red bulb reflecting into our eyes, so those areas appear bright red. Finally, when we add a blue light at different location, a new set of shadows is created and even more colors are added. Where all three colors strike the wall and reflect back we see white (the combination of the three additive primary colors- red, green and blue.) Where no light from any bulb reaches the wall we see black (the absence of light.) There are also red, green and blue areas where only those respective colors reach the wall. Finally, in spots where only red and green light reflect back we see their combination- yellow. Similarly where only red and blue is reflected back we see the combination color we call magenta (kind of a purplish-pink), and where only blue and green reflect back we see the bluish-green combination called cyan.
Thus the three colored bulbs produce a total of 6 colors on the wall (red, green, blue, yellow, cyan and magenta), eight if we include white and black (which of course we know aren't really colors.) In fact by varying the intensity of these three primary colors (red, green and blue), we can actually produce any color we want. This is why if you look closely at a TV or computer screen, you will see only red, green and blue dots or pixels. These three colors alone produce the full color images we see on the screen when we step back becuase our eyes smear and combine the pixels together.
By the way, yellow, cyan and magenta are called the secondary or complimentary colors, because they are direct combinations of each of the primary color pairs. They are also sometimes called the subtractive primary colors because they are the basic colors of paint or pigment from which all the other paint colors may be mixed (which is why artists or printers mistakenly usually refer to them as the primary colors.) When we look at yellow paint the reason that it appears yellow (under "white" or full-spectrum light illumination) is that nearly all of the blue light has been absorbed by the pigment in the paint, leaving only the green and red components of light to reflect into our eyes, where they combine to appear yellow (if you look at a yellow object in a dark room with only a blue flashlight for illumination, the object will appear black because all of the blue light is absorbed, leaving no light to reflect into your eyes.) We say the blue light has been subtracted from the "white" light, which is why yellow is called a subtractive primary color. Similarly, cyan is the color we get when red light is subtracted (absorbed), leaving only blue and green reflected back and combined. If we mix yellow and cyan pigments, now the yellow pigment absorbs the blue from our "white" light illumination, the cyan pigment absorbs the red, and only green is left to reflect into our eyes, i.e. mixing yellow and cyan paint gives us green paint. You probably learned in school to mix yellow plus blue to get green, but color purists prefer to use cyan (a bluish-green color) instead, since it allows them to better reproduce the full color spectrum they may need. If we combine each of the three subtractive color pairs in turn we get the three additive primary colors back. Try it.
Variations: Let the kids use the LED keychain flashlights from the "Writing with Light" station to combine their own light colors. Look at some ordinary objects with only one or two flashlight colors for illumination. Do they look strange?
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