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Question

A light ray I is incident on a plane mirror M. The mirror is rotated in the direction as shown in the figure by an arrow at frequency 9π rps. The light reflected by the mirror is received on the wall W at a distance 10 m from the axis of rotation. When the angle of incidence becomes 37, the speed of the spot (a point) on the wall is

A
10 m/s
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B
1000 m/s
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C
500 m/s
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D
None of these
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Solution

The correct option is B 1000 m/s
When the mirror is rotated by angle θ,


Total angle of incidence i = Total angle of reflection r=α+θ

In ΔAOB,

tan[π2(α+2θ)]=h10
i.e cot[α+2θ]=h10

Differentiating both sides,

2cosec2(α+2θ)ω=v10

Substituting α+2θ=37o,ω=2π×9π,

v=1000 m/s

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