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Light with an energy flux of 18 \mathrm{~W} \mathrm{~cm}^{-2} falls on a nonreflecting surface at normal incidence. If the surface has an area of \mathrm{20 \mathrm{~cm}^2}, the average force exerted on the surface during a 30 minute time span is

Option: 1

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


Option: 2

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


Option: 3

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


Option: 4

2.1 \times 10^6 \mathrm{~N}


Answers (1)

best_answer

The total energy falling on the surface is \mathrm{U=18 \times 20 \times(30\times 60)=6.48 \times 10^5 \mathrm{~J}}

Therefore, the total momentum delivered (for complete absorption is)

\mathrm{p=\frac{\mathrm{U}}{c}=\frac{6.48 \times 10^5}{3 \times 10^8}=\underset{2.16^{\times} 10^{-3}}{ } \mathrm{~kg} \mathrm{~m} \mathrm{~s}^{-1} .}

The average force exerted on the surface is

\mathrm{\mathrm{F}=\frac{p}{t}=\frac{2.16^{\times} 10^{-3}}{0.18 \times 10^4}=1.2 \times 10^{-6} \mathrm{~N}}

Posted by

sudhir kumar

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