Question

Electromagnetic wave of intensity $1400{\text{W/m}}^2$ falls on metal surface on area $1.5{\text{m}}^2$ is completely absorbed by it. Find out the force exerted by the beam.

• A. $14\times {10}^{-5}\text{N}$
• B. $14\times {10}^{-6}\text{N}$
• C. $7\times {10}^{-5}\text{N}$
• D. $7\times {10}^{-6}\text{N}$

Answer 1

The correct option is D $7\times {10}^{-6}\text{N}$

Force exerted,

$F=\frac{IA}{c}$

$=\frac{1400\times 1.5}{3\times {10}^8}=7\times {10}^{-6}\text{N}$

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

Why this question? Key concepts: 1. For complete absorption: $F=\frac{IA}{c}$ 2. For complete reflection in normal incidence: $F=\frac{2IA}{c}$