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  • A parallel beam of light is incident normally on a plane surface absorbing   of the light and reflecti

A parallel beam of light is incident normally on a plane surface absorbing 40 \%  of the light and reflecting the rest. If the incident beam carries 60 watt of power, the force exerted by it on the surface is
 

Option: 1

3.2 \times 10^{-8} \mathrm{~N}


Option: 2

3.2 \times 10^{-7} \mathrm{~N}


Option: 3

5.12 \times 10^{-7} \mathrm{~N}


Option: 4

5.12 \times 10^{-8} \mathrm{~N}


Answers (1)

best_answer

Momentum of incident light per second \mathrm{P_1=\frac{E}{c}=\frac{60}{3 \times 10^8}=2 \times 10^{-7}}

Momentum of reflected light per second \mathrm{P_2=\frac{60}{100} \times \frac{E}{c}=\frac{60}{100} \times \frac{60}{3 \times 10^8}=1.2 \times 10^{-7}}
Force on the surface = change in momentum per second =
\mathrm{ P_2-\left(-P_1\right)=P_2+P_1=(2+1.2) \times 10^{-7}=3.2 \times 10^{-7} N}
 

Posted by

Sanket Gandhi

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