Question

A $9\text{kW}$ light source emits light of frequency ${10}^{15}\text{Hz}$. A parallel beam of this light falls normally on a surface. If the surface completely absorbs all the incident photons, the force exerted by the light beam on the surface is

• A. $3\times {10}^{10}\text{N}$
• B. $3\times {10}^5\text{N}$
• C. $3\times {10}^{-5}\text{N}$
• D. $3\times {10}^{-9}\text{N}$

Answer 1

The correct option is C $3\times {10}^{-5}\text{N}$

Total electromagnetic energy falling on the surface per second is,

$\text{Power}=9\times {10}^3\text{W}$

Hence, the force exerted on the surface is
$F=\frac{\text{Power}}{c}=\frac{9\times {10}^3}{3\times {10}^8}=3\times {10}^{-5}\text{N}$

Hence, $\left(C\right)$ is the correct answer.