Let There Be Light!
- Monday, May 04, 2009
The combination of different wavelengths of light is referred to as additive color mixing. It is called “additive” because black is the base, and light is “added” to get to white. The combination of any two primary colors results in the secondary colors of cyan, magenta, and yellow. For example, equal intensities of red and green light combine to produce yellow. You can demonstrate this at home with colored lights or by using flashlights with colored cellophane over the lenses (you may need a couple layers of cellophane to get the desired result) shining on a white surface.
If this sounds different than what you’ve learned about mixing colors of paint, that’s because it is different! The mixing of dyes, paints, inks, and pigments is known as subtractive color mixing. It is called “subtractive” because the color we see is actually the result of either the absorption or reflection of colors by the ink or pigment. The three primary colors for subtractive color mixing are cyan, magenta, and yellow (CMY), and the secondary colors are red, green, and blue. Sound familiar? Subtractive color mixing is the basis for producing printed material, including this magazine. If you have a color printer at home, check the ink or toner colors; odds are you’ll find cyan, magenta, and yellow, plus black.
Healthy leaves on a tree in the summer look green. That is because they absorb (i.e., subtract) most of the red and blue wavelengths in the sunlight and reflect the green wavelengths. If the light shining on the green leaves has no green in it, then no green light can be reflected and the leaves will look black. In a dark room, shine a red light (or a flashlight with red cellophane over the lens) on a green leaf. What do you see?
Try This at Home
Among his many scientific contributions, Newton used lenses and prisms to show that a prism splits sunlight into its constituent colors. The conventional thinking of the day was that the prism somehow created the colors. Newton proved that the prism actually separated the colors, which added up to produce white.
Beginning science students may want to follow in Newton’s footsteps and experiment with prisms and different colors of incident light. For best results, use a good quality glass prism. You should be able to find one at a hobby shop for about $10.
Following Newton’s work with prisms, Sir Frederick William Herschel (1738–1822) made an important discovery about a kind of light that we cannot see. Herschel was one of the world’s foremost astronomers of his day and is perhaps best known for his discovery of the planet Uranus in 1781.
In 1800, while experimenting with filters for his telescopes, Herschel noticed that different color filters seemed to pass different amounts of heat. So he devised a setup for measuring the varying temperatures. This experiment can be repeated today and makes for a good science fair project.
You’ll need a good quality prism, a cardboard box, some white paper, some tape, black paint (or a black magic marker), and three thermometers. You can get the thermometers at any hardware store for less than $10 total. (Be sure to choose three thermometers that are all reading the same temperature in the store.)
First, blacken the bulbs of the thermometers so they are able to better absorb the heat from the light. Next, attach the prism to the outside edge of the box. I used a printer paper box standing on end and used the lid of the box as a base. I taped the prism in the handle hole, and I lined the lid with white paper.
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