Q45 / 54 · JEE Advanced 2019 (Paper 2) · Physics

A perfectly reflecting mirror of mass M mounted on a spring constitutes a spring-mass system of angular frequency Ω such that 4πMΩh=1024 m-2 with h as Planck's constant. N photons of wavelength λ=8π×10-6 m strike the mirror simultaneously at normal incidence such that the mirror gets displaced by 1 μm. If the value of N is x×1012, then the value of x is________.
[Consider the spring as massless]
Figure

Correct answer: 1

Solution

Total Momentum transferred on mirror =2Nhλ (for perfect reflection)
In an SHM
Vmax=A.Ω
Where A= amplitude and Ω= angular frequency
⇒2Nhλ=MVmean position
Vmean position=ΩAwhere A=1 μm
⇒2Nhλ=MΩAwhere λ=8π×10-6
∴N=λMΩA2h
⇒N=MΩ10-6λ2h=MΩ8π×10-6×10-62h
N=4πMΩh×10-12
=1024×10-12
N=1×1012
x=1

Hence, the correct answer is 1.

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