Nature has exhibited the effects of a variable index of refraction for a very long time. The index of refraction of air varies with its density. Therefore, the index of refraction of the Earth’s atmosphere decreases with altitude, and light rays bend toward the vertical as they approach the ground. As a consequence, the Moon appears about one diameter higher than it actually is when it is near the horizon. Temperature also affects the density of air, and this dependence creates the mirages we see on road surfaces as we drive down highways on hot days. Identical effects occur with other types of wave—for instance, sound waves—which creates the basis for this chapter.