A $9 kW$ light source emits light of frequency $10^{15} Hz$ . A parallel beam of this light falls normally on a surface. If the surface completely absorbs all the incident photons, the force exerted by the light beam on the surface is

3 \times 10^{10} N

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3 \times 10^{5} N

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3 \times 10^{- 5} N

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3 \times 10^{- 9} N

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Solution

The correct option is C $3 \times 10^{- 5} N$
Total electromagnetic energy falling on the surface per second is,

$Power = 9 \times 10^{3} W$

Hence, the force exerted on the surface is
$F = \frac{Power}{c} = \frac{9 \times 10^{3}}{3 \times 10^{8}} = 3 \times 10^{- 5} N$

Hence, $(C)$ is the correct answer.

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