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Question

Find the angle of incidence (θ) of a paraxial ray incident on a concave mirror of large aperture, for which it passes through the pole (P) after reflection.

A
60
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B
30
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C
45
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D
75
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Solution

The correct option is A 60
For the concave mirror, C is the centre of curvature and CI is the normal direction.

From laws of reflection, i=r
MIC=CIP=θ
From alternate angle, ICP=MIC=θ
For paraxial rays,
CI=CP=R (radius of curvature)
CIP=IPC=θ
[ in a triangle, angles opposite to equal sides are equal ]
In CIP,
CIP+ICP+IPC=180
θ+θ+θ=180
θ=60

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