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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 2.50×10−6 NGiven:- Intensity of light, I=25×104 W/m2 Also, I=UA⋅Δt ..........(1) Now, rate of change in momentum of light after being reflected from a perfectly reflecting surface is ΔpΔt=2 Uc⋅Δt Average force exerted on the surface is given by F=ΔpΔt=2 Uc⋅Δt=2IAΔtcΔt ∴F=2IAc F=2×25×104×15×10−43×108 F=250×10−8 N ∴F=2.5×10−6 N Hence, option (B) is correct.

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