Question

Light with an average flux of $20{\text{W/cm}}^2$ falls on an absorbing surface at normal incidence having surface area $20{\text{cm}}^2$. The momentum received by the surface during time span of $1$ second is

• A. $12\times {10}^3\text{N s}$
• B. $24\times {10}^3\text{N s}$
• C. $48\times {10}^3\text{N s}$
• D. $1.33\mu \text{N s}$

Answer 1

The correct option is D $1.33\mu \text{N s}$

Momentum due to electromagnetic wave is given by

$p=\frac{U}{c}$ where $U$ is energy

$=\frac{IA\Delta t}{c}=\frac{20\times 20\times 1}{3\times {10}^8}=1.33\mu \text{N s}$