Question

A $1.5kW$ (kilo-watt) laser beam of wavelength $6400\mathring{A}$ is used to levitate a thin aluminium disc of same area as the cross section of the beam. The laser light is reflected by the aluminium disk without any absorption. The mass of the disc is close to:

• A. ${10}^{-9}kg$
• B. ${10}^{-3}kg$
• C. ${10}^{-4}kg$
• D. ${10}^{-6}kg$

Answer 1

Answer: (D)

${10}^{-6}kg$

Let the power of the beam be P.

Force acting on the disc is given by:

$F=\frac{2P}{c}=\frac{2\times 1.5\times {10}^3}{3\times {10}^8}={10}^{-5}N$

The above force balances the weight of the disc. Hence,

$F=mg$

${10}^{-5}=10g$

$m={10}^{-6}kg$