What we call light is really just a tiny portion of the whole spectrum of radiant energy, which includes radio waves, television waves, and X rays. Light is the part of the spectrum that we have receptor systems—our eyes—to detect. As you can see from the figure below, the visual spectrum consists of electromagnetic energy with wavelengths from about 350 to 750 nanometers (nm). (A nanometer is one-billionth of a meter.)

Reading the colors in the spectrum from right to left gives us the familiar name “ROY G BIV,” which is a mnemonic, or memory, device enabling us to remember that the colors of the visual spectrum are Red, Orange, Yellow, Green, Blue, Indigo, and Violet, in that order.

As you may remember from another course, white light is a mixture of all colors. One thing you need to keep in mind in this discussion is that there are two types of color mixtures—additive and subtractive. A subtractive color mixture is what you get when you mix pigments. For example, if you mixed red and green paint, you would produce paint with the color brown. On the other hand, if you mix red light and green light—an additive color mixture—you get the color yellow. When we talk about color vision phenomena, we are talking about an additive color mixture.