The physicist Richard Feynman once illustrated the extraordinary nature of vision as follows: We are immersed in a sea of electromagnetic waves whose lengths vary over a huge range (Figure 1.1). These waves interact with each other and with objects around us to present a cacophony of electromagnetic signals to our eyes. Through a tiny aperture, about 2 mm in diameter, the eye selects a small fraction of these wavelengths and, together with the brain, reconstructs the position, shape, color, and motion of each object we see around us. Feynman compared the situation to that of a water bug floating on the surface at one corner of a swimming pool. The only information available to the bug comes from the movements of its body caused by the waves that reach it. Were the bug able to reconstruct from these waves the positions and motions of all the people entering, leaving, and swimming in the pool, it would be doing something similar to what the eye and brain do with the minuscule electromagnetic disturbances passing through the pupil.

Why does the eye normally respond only to electromagnetic energy with wavelengths in the range 400–700 nm (1 nm = 10-9 m)? To answer this question, one must ask what electromagnetic energy was available to the earliest living forms that developed vision. The major source of such energy reaching the surface of the earth is the sun, which emits radiation with the spectrum shown in Figure 1.2.