Question

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

Answer 1

We know,
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 I,
Force = $\frac{\pi r^{2}I}{C}$
$I=0.5$ $W/m^2,$ $r=1cm,$ $C=3\times {10}^{8}m/s$
Force = $\frac{\pi \times {\left(1\right)}^2\times 0.5}{3\times {10}^8}=\frac{3.14\times 0.5}{3\times {10}^8}$
$=0.523\times {10}^{-8}=5.2\times {10}^{-9}N.$