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 0.5 W cm−2. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.

Answer 1

The force exerted by the light beam on the sphere is $5.2 \times 10^{-9} N$

Explanation:

Given data in the question  

The sphere radius,r = 1 cm

Light intensity, $I=0.5 \mathrm{W} \mathrm{cm}^{-2}$

Let A be the sphere's effective area perpendicular to the light radius.

Thus force exerted on the sphere by the light beam is given,

$F=\frac{P}{c}=\frac{A I}{c}$

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

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

$=0.523 \times 10^{-8}$  

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