A plate of mass $10g$  is in equilibrium in air due to the force exerted by light beam on plate. Calculate power of beam. Assume plate is perfectly absorbing.

 

For the plate to be in equilibrium, the force due to absorbed light radiation should equal the weight of the plate.

If P = power of light beam, then

$$\begin{gathered} \frac{P}{c}=mg \\ \Rightarrow P=mgc=\left(10\times {10}^{-3}\right)\times \left(10\right)\times \left(3\times {10}^8\right)=3\times {10}^7 \mathrm{W} \end{gathered}$$