Question

Light with an energy flux of $25\times {10}^4{\text{Wm}}^{-2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15{\text{cm}}^2$, the average force exerted on the surface is :

• A. $1.25\times {10}^{-6}\text{N}$
• B. $2.50\times {10}^{-6}\text{N}$
• C. $1.15\times {10}^{-6}\text{N}$
• D. $3.0\times {10}^{-6}\text{N}$

Answer 1

The correct option is B $2.50\times {10}^{-6}\text{N}$

For complete reflection in normal incidence, the average force exerted by the beam will be,

$F=\frac{2IA}{c}$

$=\frac{2\times 25\times {10}^4\times 15\times {10}^{-4}}{3\times {10}^8}=2.50\times {10}^{-6}\text{N}$

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