At a given point in its orbit, the moon is 2.4 x 105miles from earth. How long does it take light from a source on earth to reach a reflector on the moon and then return to earth

2.47 seconds.

Explanation:

We know that at this point, the distance between the moon and Earth is:

D = 2.3*10^5 miles

We want to know how much time does need the light to go from Earth to the moon and then return to Earth.

Here we can use the relationship:

Time = Distance/Speed

Distance is equal to two times the distance between Earth and the moon (because the path is traveled two times), then:

Distance = 2*(2.3*10^5 miles) = 4.6*10^5 miles.

The speed is equal to the speed of light, we know that this is:

Speed = 186,282 mi/s

Then the time will be:

time = (4.6*10^5 mi)/( 186,282 mi/s) = 2..47 s

The light needs 2.47 seconds to go from Earth to the moon and come back.

the part of the wave he is travelling on is the crest