Q122 / 125 · BITSAT 2020 · Physics

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 $)$

  • A
    zero
  • B
    $\mathrm{E} / \mathrm{Atc}$
  • Correct answer
    C
    $2 \mathrm{E} / \mathrm{Atc}$
  • D
    $\mathrm{E} / \mathrm{c}$

Solution

Incident momentum, $\mathrm{p}=\frac{\mathrm{E}}{\mathrm{c}}$ For perfectly reflecting surface with normal incidence $\Delta \mathrm{p}=2 \mathrm{p}=\frac{2 \mathrm{E}}{\mathrm{c}}$ $\mathrm{F}=\frac{\Delta \mathrm{p}}{\Delta \mathrm{t}}=\frac{2 \mathrm{E}}{\mathrm{ct}}$ $\mathrm{P}=\frac{\mathrm{F}}{\mathrm{A}}=\frac{2 \mathrm{E}}{\mathrm{ctA}}$

Hence, the correct answer is option: 'C'.

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