Light can be described as a wave, and we perceive different wavelengths of light as different colors. For example, the wavelength of a photon of blue light is only forty thousandths of a millimeter, or 0.4 µ, whereas a red photon has a longer wavelength, about 0.7 µ.

There are two different kinds of cells in our eyes: those sensitive only to the intensity of light, resulting in b&w vision, and other cells that are sensitive to color — in other words, sensitive to the different wavelengths of light. Have you ever noticed that in dim light, you can't see any color? That's because the color cells in your eyes are not as sensitive as the other cells and so they need brighter light to respond. However, if you go out at night to a dark place and wait about 20 minutes, your color-eye cells will become more sensitive. And patience pays off because you'll start to notice that some stars are red, while others are blue, white and yellow.