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Question

Light with an energy flux of 25×104 W/m2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm2, the average force exerted on the surface is

A
1.25×106 N
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B
2.50×106 N
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C
1.15×106 N
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D
3.50×106 N
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Solution

The correct option is B 2.50×106 N
Given:-
Intensity of light, I=25×104 W/m2

Also, I=UAΔt ..........(1)

Now, rate of change in momentum of light after being reflected from a perfectly reflecting surface is

ΔpΔt=2 UcΔt

Average force exerted on the surface is given by

F=ΔpΔt=2 UcΔt=2IAΔtcΔt

F=2IAc

F=2×25×104×15×1043×108

F=250×108 N

F=2.5×106 N

Hence, option (B) is correct.

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