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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.

Q 2.29: 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 ?

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2.56 s is the total time taken by the LASER to reach Moon and again back Earth.

\therefore Time taken by LASER to reach Moon = \frac{1}{2}\times 2.56 s = 1.28 s

We know, Speed of light \approx 3\times10^8 ms^{-1}

\therefore The radius of the lunar orbit around the Earth = distance between Earth and Moon = 

= Speed of light x Time taken by laser one-way = 3\times10^8\ ms^{-1}\times1.28\ s = 3.84\times 10^8 \ m

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