Question

A point source of light is placed at the centre of curvature of a hemispherical surface The source emits a power of $24\, W$ The radius of curvature of hemisphere is $10\, cm$ 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

Force =$∫\;Pd\;A\;cosθ$

= $\frac{2I}{C}​∫dA\;cosθ = \frac{2I}{C}​πR^2= 2 \frac{p_o}{4\pi R^2}.\frac{\pi R^2}{C}$​

= $\frac{p_0}{2C}​​=\frac{24}{2\times3\times10^8}​=4×10^{−8}N$

The correct answer is 4.