Chapter 56
Color
The human eye is capable of both color and black-and-white vision. Under conditions of very low illumi-
nation, a set of very light-sensitiverodson the retina of the eye allow us to see in black and white. Under
higher illumination, a different set of light receptors calledconesbecome active that permit color vision. The
retina contains three types of cones, each of which is mostly sensitive to a different color: red, green, and
blue. Combinations of these three primary colors allow us to see all the other colors.
What we perceive as “white” light is actually a combination of all colors of light. We can split white light
into its component colors using a prism or a diffraction grating; the resulting colors and their approximate
wavelengths are shown in Table 56-1. (The sequence of colors can be remembered from the mnemonic ROY
G. BIV.)
Table 56-1. Approximate wavelengths of colors in the spectrum.Color Wavelength (nm)
Red 650
Orange 590
Yellow 570
Green 510
Blue 475
Indigo 445
Violet 400Our perception of color is a complicated process. It depends partly on the wavelength of light received by
the eye; but also the brain is able to distinguish colors by comparing the brightness of an object to other nearby
objects, as seen by all three colors of cones on the retina. This complicated process (called theLand effect)
allows us to perceive objects to be the same color, even under very different lighting conditions. (Notice,
for example, that objects appear to have the same color indoors under a fluorescent light as they do outdoors
under sunlight.) This phenomenon is calledcolor constancy.
56.1 Lights
There are threeprimary colorsof light:red,green, andblue. Other colors of light can be made by combining
these three primary colors in different proportions. Equal proportions of red and green light makeyellow