Rainbow Shadows

Imagine you’re a painter. What three colors do you need to make up any color in the universe? (You should be thinking: red, yellow, and blue… and yes, you are right if you’re thinking that the real primary colors are cyan, magenta, and yellow, but some folks still prefer to think of the primary colors as red-yellow-blue… either way, it’s really not important to this experiment which primary set you choose.)

Here’s a trick question – can you make the color “yellow” with only red, green, and blue as your color palette? If you’re a scientist, it’s not a problem. But if you’re an artist, you’re in trouble already.

The key is that we would be mixing light, not paint. Mixing the three primary colors of light gives white light. If you took three light bulbs (red, green, and blue) and shined them on the ceiling, you’d see white. And if you could magically un-mix the white colors, you’d get the rainbow (which is exactly what prisms do.)

If you’re thinking yellow should be a primary color – it is a primary color, but only in the artist’s world. Yellow paint is a primary color for painters, but yellow light is actually made from red and green light. (Easy way to remember this: think of Christmas colors – red and green merge to make the yellow star on top of the tree.) It’s because you are using projection of light, not the subtrative combination of colors to get this result.

Here’s a nifty experiment that will really bring these ideas to life (and light!)

Materials

flashlight (three is best, but you can get by with two)
fingernail polish (red, green, and blue)
clear tape or cellophane (saran wrap works too)
white wall space

Here’s what you do: Find three flashlights. Cover each with colored cellophane (color filters) or paint the plastic lens cover with nail polish (red, green, and blue). Shine onto a white ceiling or wall, overlap the colors and make new colors. Leave the flashlights on, line them up on a table, turn off the lights, and dance – you will be making rainbow shadows on the wall! In addition, you can paint the lens of a fourth flashlight yellow to see what happens.

When you combine red and green light, you will get yellow light. Combine green and blue to get cyan (turquoise). Combine blue and red to get magenta (purple). Turn on the red and green lights and the wall will appear yellow. Wave your hand in front of the lights and you will see cyan and magenta shadows. Turn on the green and blue lights, and the wall turns cyan with yellow and magenta shadows. Turning on the blue and red give a magenta wall with yellow and cyan shadows. Turn on all colors and you will get a white wall with cyan, yellow, and magenta shadows – rainbow shadows!

Troubleshooting: This experiment has a few things to be aware of. If you’re not getting the colored shadows, check to be sure that the flashlight is bright enough to illuminate a wall in the dark. Be sure to shut the doors, shades, windows, and drapes. In the dark, when you shine your red flashlight on the wall, the wall should glow red. Beware of using off-color nail polish – make sure it’s really red, not hot pink.

If you still need help making this experiment work, you can visit your local hardware store and find three flood lamp holders (the cheap clamp-style ones made from aluminum work well – you’ll need three) and screw in colored “party lights” (make one red, one green, and one blue), which are colored incandescent bulbs. These will provide a lot more light! You can also add a fourth yellow light to further illustrate how yellow light isn’t a primary color. Try using only red, yellow, and blue… you’ll quickly find that you can’t obtain all the colors as you could with the original red-green-blue lights.

Exercises

What are the three primary colors of light?
What color do you get when mixing the primary colors of light?
How do you mix the primary colors of light to get yellow?
Use crayons or colored pencils to draw what you saw when all three lights were shining on the wall and you waved your hand in front of the light.