A light beam of intensity 20 ,W/cm^2 is incid | vedclass

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(C) Given: Intensity $I = 20 \,W/cm^2 = 20 	imes 10^4 \,W/m^2$. Area $A = 25 \,cm 	imes 15 \,cm = 375 \,cm^2 = 375 	imes 10^{-4} \,m^2$. Time $t =

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$5 	imes 10^{-5} \,kg \cdot m/s$

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(C) Given: Intensity $I = 20 \,W/cm^2 = 20 	imes 10^4 \,W/m^2$. Area $A = 25 \,cm 	imes 15 \,cm = 375 \,cm^2 = 375 	imes 10^{-4} \,m^2$. Time $t = 1 \,s$. The energy $E$ incident on the surface per second is $E = I 	imes A 	imes t = 20 	imes 10^4 	imes 375 	imes 10^{-4} 	imes 1 = 7500 \,J$. For a perfectly reflecting surface,the momentum $p$ imparted by light is given by $p = \frac{2E}{c}$,where $c = 3 	imes 10^8 \,m/s$ is the speed of light. $p = \frac{2 	imes 7500}{3 	imes 10^8} = \frac{15000}{3 	imes 10^8} = 5000 	imes 10^{-8} = 5 	imes 10^{-5} \,kg \cdot m/s$.

$A$ light beam of intensity $20 \,W/cm^2$ is incident normally on a perfectly reflecting surface of sides $25 \,cm \times 15 \,cm$. The momentum imparted to the surface by the light per second is

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