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 A60∘ 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∘