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?

Question

Philoid · Accepted Answer

Given,

Time taken by laser to reflect back from moon to earth = 2.56 s

We know that,

Speed of the light in vacuum = 3×10⁸ m/s

Radius of the lunar orbit (r) = distance between Earth and Moon (d)

Times taken for laser beam to reach moon = 0.5×2.56 = 1.28 s

∴ r = d = speed of light in vacuum × Time taken

= 3×10⁸ × 1.28 m

= 3.84×10⁵ km

Hence, the radius of the lunar orbit is, 3.84×10⁵ km.