When we see different colors, we are perceiving differences in the light that is reaching our eyes. The way we see different colors is something like the way we hear different sounds as being "low" or "high." This is called pitch, and it corresponds to the frequency of the sound.
The keys on the left side of a piano keyboard make low-frequency sounds, for example, and the frequency of the sound gets higher as one plays keys further to the right. There is a similar order to the colors we see.
The colors of every rainbow always appear in the same order: red, orange, yellow, green, blue, indigo, and violet. The different colors in each part of the rainbow correspond to a different wavelength of light. Reddish colors are long in wavelength and bluish colors are shorter. And just as there are many notes on the piano, there are many wavelengths of light corresponding to different colors.
How Does the Eye Normally See Colors?
Think of your eye as a camera. The front of the eye contains a lens that focuses images on the inside of the back of the eye. This area, called the retina, is covered with special nerve cells that react to light.
These retinal nerve cells include the rods and cones. The rods and cones react to light because they contain pigments that change when light strikes them.
The cones are responsible for color vision. There are several kinds of pigments present in three types of cone cells. Some cones react to short-wavelength light, others react to medium wavelengths, and others react to higher wavelengths.
There is only one kind of pigment in the rods, and it reacts the same way to any wavelength of light. The rods do not have anything to do with color vision; however, they are very sensitive to light and allow us to see at night.
When the cones have all the various pigments, the eye sees all possible colors. It is something like the way a painter can mix just a few colors together and make paint of every possible color.