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Question 29

A LASER is a source of very intense, monochromatic, and unidirectional beam of light. These properties of a laser light can be exploited to measure long distances. The distance of the Moon from the Earth has been already determined very precisely using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moon's surface. How much is the radius of the lunar orbit around the Earth?


Time taken by the laser beam to return to Earth after reflection from the Moon = 2.56 s

Speed of light = 3 × 108 m/s

Time taken by the laser beam to reach Moon = 2..56 x 1/2 = 1.28 s

Radius of the lunar orbit = Distance between the Earth and the Moon = 1.28 × 3 × 108 = 3.84 × 108 m = 3.84 × 105 km

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