Ncert Exercises & Solutions

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?
1. \(3.84\times 10^{8}~\text m\)
2. \(1.28\times 10^{8}~\text m\)
3. \(7.68\times 10^{8}~\text m\)
4. \(2.56\times 10^{8}~\text m\)

We known that speed of laser light = c = 3 x 10⁸m/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 h e n,

t = 2.56 = \frac{2 d}{c}

\Rightarrow d = \frac{1}{2} \times 2.56 \times 3 \times 10^{8} = 3.84 \times 10^{8} m

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