Chapter 9, Problem 2C

In Section 9.6, we described how the speed of light varies with wavelength (or frequency) for transparent solids. But the speed of light in matter is also a function of temperature and pressure. This dependence is most marked for gases and is instrumental in producing such things as mirages and atmospheric refraction, the latter phenomenon being the displacement of an astronomical object (like the Sun or another star) from its true position because of the passage of its light through the atmosphere. Because Earth’s atmosphere is a gaseous mixture and easily compressed, its density is highest near Earth’s surface and gradually declines with altitude. (Refer to the discussion in Section 4.4 and Figure 4.29.) Thus, the speed of light in the atmosphere is lowest near the surface and gradually gets higher, approaching c as one goes farther and farther into space. Using this fact and the law of refraction, sketch the path a light ray from the Sun would follow upon entering Earth’s atmosphere, and predict the apparent position of the Sun relative to its true position (Figure 9.85). What does this tell you about the actual location of the Sun’s disk relative to your local horizon when you see it apparently setting brilliantly in the west in the evening?