Conceptual Questions Chap. 22.
The spectrum of the light sent back to you from a drop at the top of the rainbow arrives such that the red light strikes the eye while the violet light passes over your head. At the bottom of the bow, violet light arrives at your eye and red light is deviated toward the ground (see Figure 22.20).
A mirage occurs when light changes direction as it moves between batches of air having different indices of refraction, due to air having different densities at different temperatures. Two images are seen: one from a direct path from the object to you, and the second arriving by rays originally heading toward the ground but refracted to your eye. On a hot day, the Sun makes the surface of blacktop hot, so the air is hot directly above it, becoming cooler with increasing altitude. The “water” we see is an image of the blue sky. Shimmering occurs when the air changes in temperature.
The index of refraction of water is quite different from air, while the index of refraction of liquid helium happens to be much closer to that of air. Consequently, light undergoes less refraction at the helium-air interface than it does at the water-air one.
The diamond acts like a prism in dispersing the light into its spectral components.