Question

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 _____ $\times 10^{-8}N$

Answer 1

Answer: 4

$\text{ Force }=\int PdA\cos \theta$
$=\frac{2I}{C}\int dA\cos \theta =\frac{2I}{C}\pi R^2=2\frac{p_0}{4\pi R^2}\cdot \frac{\pi R^2}{C}$
$=\frac{p_0}{2C}=\frac{24}{2\times 3\times 10^8}=4\times 10^{-8}N$