During the Apollo XI Moon landing, a retro- reflecting panel was erected on the

Question

During the Apollo XI Moon landing, a retro-
reflecting panel was erected on the Moon’s
surface. When the moon is directly ahead, the
speed of light is found by measuring the time
it takes a laser beam to travel from Earth,
reflect from the screen, and return to Earth.
If this interval is measured to be 2.5 s, what
is the measured speed of light? Take the
center-to-center distance from Earth to Moon
to be 3.72 × 10^8 m, and do not neglect the
radii of the Earth (6.37 × 10^6 m) and Moon
(1.74 × 10^6 m).
Answer in units of m/s.

Explanation / Answer

Surface to surface distance between moon and Earth = (3.72E8) - [radius of Earth + radius of moon] = (3.72E8) - (6.37E6 + 1.74E6) = 3.64E8 m The distance traveled by laser beam = 2 x 3.64E8 = 7.28E8 m Time taken = 2.5 seconds Speed of light = 7.28E8 m / 2.5 seconds = 2.91E8 m/s

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