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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?

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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 = (1/2) x 2.56 = 1.28s

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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