Question

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?

Answer 1

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 xx 2.56 = 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

Answer 2

We known that speed of laser light = c = 3 x 10⁸ m/s. If d be the distance of Moon from the earth, the time taken by laser signal to return after reflection at the Moon’s surface

t =2.56 s = `(2d)/c = (2d)/(3xx10^8 ms^(-1))`

`=> d = 1/2 xx 2.56 xx 3 xx 10^(8) m = 3.84 xx 10^8 m`