Light with an energy flux of $25 \times 10^{4} {Wm}^{- 2}$ falls on a perfectly surface at normal incidence. If the surface area is $15 {cm}^{2}$ , the average force exerted on the surface is

1.25 \times 10^{- 6} N

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2.50 \times 10^{- 6} N

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1.20 \times 10^{- 6} N

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3.0 \times 10^{- 6} N

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Solution

The correct option is B $2.50 \times 10^{- 6} N$
Average force for reflecting surface $\frac{Δ p}{Δ t} = \frac{2 I A}{C}$

$= \frac{2 \times 25 \times 10^{4} \times 15 \times 10^{- 4}}{3 \times 10^{8}}$

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

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