Question 136 of 200

A point source of light is placed at the centre of curvature of a hemispherical surface. The source emits a power of $24W$. The radius of curvature of hemisphere is $10cm$ and the inner surface is completely reflecting. The force on the hemisphere due to the light falling on it is ______ $\times {10}^{-8}N$.

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Answer: 4

For completely reflecting surface, force, $F=\dfrac{2IA}{c}$, where, $I=\dfrac{P}{Surfacearea}$ is intensity of radiation, $A$ is surface area and $c$ is speed of light.
$F=\dfrac{2I}{c}\pi {R}^{2} =2\dfrac{{P}_{0}}{4\pi {R}^{2}}\dfrac{\pi {R}^{2}}{c}$
$=\dfrac{{P}_{0}}{2c} =\dfrac{24}{2\times 3\times {10}^{8}} =4\times {10}^{-8}N$