Question

A sphere of radius 1.00 cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is 0.50 W $c{m}^{-2}$. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.

Answer 1

If a perfectly reflecting solid sphere of radius 'r' is kept in the path of a parallel beam of light of large aperture if intensity is l,

Force = $\frac{\pi r^{2}l}{c}$

$l=0.5\frac{W}{m^2}$

$r=1\text{cm,}c=3\times {10}^8\frac{m}{s}$

Force = $\frac{\pi \times (1{)}^2\times 0.5}{3\times {10}^8}$

$\frac{3.14\times 0.5}{3\times {10}^8}$

$=0.523\times {10}^{-8}$

$=5.2\times {10}^{-9}N$