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  • Light with an energy flux of falls on a non-reflecting surface at normal incidence. If surface has

Light with an energy flux of 18 \ W/cm^2falls on a non-reflecting surface at normal incidence. If surface has an area of 20 \mathrm{~cm}^2, the average force exerted on surface in a duration of 30 minutes is

Option: 1

0.6 \times 10^{-6} \mathrm{~N}


Option: 2

0.9 \times 10^{-6} \mathrm{~N}


Option: 3

1.2 \times 10^{-6} \mathrm{N}


Option: 4

2.4 \times 10^{-6} \mathrm{~N}


Answers (1)

Total energy incident on the given surface in a time interval of 30 \mathrm{~minutes} is
\begin{aligned} E & =18 \times 10^4 \times 20 \times 10^{-4} \times 30 \times 60 \\ & =648 \times 10^5 \mathrm{~J} \end{aligned}

Therefore the total momentum transferred to a given surface for complete absorption is
\Delta p=\frac{E}{c}=\frac{648 \times 10^5}{3 \times 10^8}=2.16 \times 10^{-3} \mathrm{~kg}-\mathrm{m} / \mathrm{s}
Average force F_{a v}=\frac{\Delta P}{t}=\frac{2.16 \times 10^{-3}}{30 \times 60}
F_{a v}=1.2 \times 10^{-6} \mathrm{~N}

 

Posted by

Sumit Saini

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