Question

A parallel beam of monochromatic light of wavelength $500nm$ is incident normally on a perfectly absorbing surface. The power through any cross-section of the beam is $10W$. What is the force exerted by it?

• A. $6.67\times {10}^{-8}N$
• B. $3.33\times {10}^{-8}N$
• C. $2\times {10}^{-8}N$
• D. $5\times {10}^{-8}N$

Answer 1

The correct option is B $3.33\times {10}^{-8}N$

For perfectly absorbing surface,
Pressure $P=\frac{\text{Intensity}}{\text{Speed of light}}=\frac{I}{c}$

since, force $F=P\times A=\frac{I\times A}{c}$ ; we know $I=\frac{P}{A}$

$=\frac{\text{Power}}{c}=\frac{10}{3\times {10}^8}=3.33\times {10}^{-8}N$