Question

A light beam of intensity 0.5 $W/m^2$ encounters a cone on its path as shown in the figure. Radius of cone is 1 m and height is 2 m. If surface of cone is perfectly absorbing, then force exerted on the cone by falling light is $n\times {10}^{-9}N$ here n is
(Write upto two digits after the decimal point.)

Answer 1

$I=0.5$, $A=\frac{1}{2}2r\times h=2m^2$
Momentum, $P=\frac{h}{\lambda }=\frac{hc}{\lambda c}=\frac{E}{c}$
Force, $F=\frac{dP}{dt}=\frac{1}{c}\frac{dE}{dt}=\frac{IA}{c}=\frac{0.5\times 2}{3\times {10}^8}=3.33\times {10}^{-9}N$