Question

A laser beam $\lambda =633nm$ has a power of $3mW$. What will be the pressure exerted on a surface by this beam if the cross-section area is $3m{m}^2$? (Assume perfect reflection and normal incidence).

• A. $6.6\times {10}^{-3}\frac{N}{m^2}$
• B. $6.6\times {10}^{-6}\frac{N}{m^2}$
• C. $6.6\times {10}^{-9}\frac{N}{m^2}$
• D. $6.6\frac{N}{m^2}$

Answer 1

The correct option is B $6.6\times {10}^{-6}\frac{N}{m^2}$

Pressure on surface if beam is perfectly reflected $=\frac{2\times \text{Intensity}}{c}$

Here, intensity of beam $=\frac{\text{Power}}{\text{Area}}$
$=\frac{3\times {10}^{-3}}{3\times {10}^{-6}}={10}^{3}W{m}^{-2}$

$\Rightarrow \text{Pressure}=\frac{2\times {10}^3}{3\times {10}^8}=6.6\times {10}^{-6}N{m}^{-2}$