Question

Chapter 24, Problem 21 GO

A mirror faces a cliff located some distance away. Mounted on the cliff is a second mirror, directly opposite the first mirror and facing toward it. A gun is fired very close to the first mirror. The speed of sound is 335 m/s. How many times does the flash of the gunshot travel the round-trip distance between the mirrors before the echo of the gunshot is heard?

Answer 1

The light from the flash travels a round trip of 2d "n" times, while the sound travels one round trip

distance for light = speed oflight * time or n 2d = v_Lt

distance for sound = speed ofsound * time or 2d =V_s *t

If you take the ratio of these expressions, the value of n,which is what you want, is just

n = speed of light / speed of sound

= 300000000 m/s / 335m/s

=895522.39

= 875523   

The light is this must faster than sound, so it travels this many round trips when the sound makes just one round trip.