As you probably know... a given color of light can generally, be described by one number (e.g., the frequency of its wave length).

The color white, however, has no associated frequency; it is composed of equal amounts of all the (visible) frequencies of light. Black, of course, is the absence of light. And various shades of grey are generally composed of various intensities of white (as you lower the overall intensity of white, the grey gets darker and approaches black).

The question is: why is light (optically) additive? More specifically, why is it that when we add Red and Green together we get Yellow; Green and Blue together, we get Cyan; Red, Green, and Blue together, we get white?

(Please don't confuse this with the subtractive quality of pigments/paints, where as we add more colors, particularly more of the primary colors red, yellow, and blue, we approach black.)

Could we use three (or more) different colors and achieve the same result?

Objects do not have color. They are only coated with a substance that reflects light of a certain wavelength. Assuming they are illuminated with white light, coating (A) will reflect red light stronger than the other wavelengths, which are still present in varying degrees, but overpowered. Coating (B) green. A mixture of both coatings will be seen to reflect yellow because of destructive interference.The red wavelengths cancel out the green, leaving yellow as the strongest remaining reflective wavelength. Using lasers for illumination in our experiment, this is not the case. A red helium-neon laser, and a green argon laser mixed together will not produce yellow light. So light is not additive after all.

Posted by john on 2004-03-05 02:10:43