Question

Falling light. According to general relativity and the equivalence principle, light is bent by gravity. Consider two tall, perfectly reflecting mirrors exactly 7 m apart and facing each other. A beam of light is directed horizontally through a hole in one of the mirrors 12 m above the ground.

(a) Determine the time it takes for the light to strike the ground.

(b) The light will undergo N reflections (i.e., N/2 reflections from each mirror) before it strikes the ground. Find N.

(Please make numbers legible and with units)

Answer 1

Assuming the mirrors are perfectly parallel, and otherwise vertical, the answers are:
a) never or infinity, since light will go right back out the hole it came in on. And if it didn't, then the mirrors will rotate with the Earth, and the beam will appear to climb up and out of the mirrors (depending on orientation of the reflecting plane to Earth's rotational axis, and location with respect to the equator).

b) N = 1 or infinity, since the ground is never hit.