# Light with an energy of $25\times 10^{4} Wm^{-2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 $cm^{2}$, the average force exerted on the surface is:- Option 1) $1.25\times 10^{-6}N$ Option 2) $2.50\times 10^{-6}N$ Option 3) $1.20\times 10^{-6}N$ Option 4) $3.0\times 10^{-6}N$

S subam

Momentum of EM wave -

$p = \frac{u}{c}$

- wherein

u = Energy of EM wave

c = Speed of light in vacuum

For a perfectly reflecting surface

$\Delta p=2p$

$\Delta p=\frac{2E}{C}\\ Force F=\frac{\Delta p}{\Delta t}=\frac{2\times power}{C}=\frac{2\times 25\times 10^{4}}{3\times 10^{8}}\times 15\times 10^{-4}\\ F=2.5\times 10^{-6}N$

Option 1)

$1.25\times 10^{-6}N$

This solution is incorrect

Option 2)

$2.50\times 10^{-6}N$

This solution is correct

Option 3)

$1.20\times 10^{-6}N$

This solution is incorrect

Option 4)

$3.0\times 10^{-6}N$

This solution is incorrect

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