Discussion
A laser beam of intensity 50 W/cm\(^{2}\) falls on a perfectly reflecting plane mirror for an hour. The area of the mirror is 5 cm\(^{2}\). The average force acting on the mirror is few :
Answer: C
The force is a few Micro Newtons.
The power falling on the mirror is \(P = IA = 50 \times 5 = 250\) W. For perfect reflection the momentum of the light is reversed, so the force is twice P/c:
\[ \begin{aligned} F &= \frac{2P}{c} = \frac{2 \times 250}{3 \times 10^{8}} \\ &\approx 1.7 \times 10^{-6}\ \text{N} \end{aligned} \]
That is about 1.7 micronewtons. The duration of one hour does not affect the force, only the total momentum transferred.
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