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A plane electromagnetic wave is incident on a plane surface of area A, normally and is perfectly reflected. If energy E strikes the surface in timet then average pressure exerted on the surface is $(\mathrm{c}=$ speed of light $)$
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$2 \mathrm{E} / \mathrm{Atc}$
Incident momentum, $\mathrm{p}=\frac{\mathrm{E}}{\mathrm{c}}$ For perfectly reflecting surface with normal incidence
$\Delta \mathrm{p}=2 \mathrm{p}=\frac{2 \mathrm{E}}{\mathrm{c}}$
$\mathrm{F}=\frac{\Delta \mathrm{p}}{\Delta \mathrm{t}}=\frac{2 \mathrm{E}}{\mathrm{ct}}$
$\mathrm{P}=\frac{\mathrm{F}}{\mathrm{A}}=\frac{2 \mathrm{E}}{\mathrm{ctA}}$
$\Delta \mathrm{p}=2 \mathrm{p}=\frac{2 \mathrm{E}}{\mathrm{c}}$
$\mathrm{F}=\frac{\Delta \mathrm{p}}{\Delta \mathrm{t}}=\frac{2 \mathrm{E}}{\mathrm{ct}}$
$\mathrm{P}=\frac{\mathrm{F}}{\mathrm{A}}=\frac{2 \mathrm{E}}{\mathrm{ctA}}$
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