Q.

Define focal length of a concave mirror. Prove that the radius of curvature of a concave mirror is double its focal length.

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Detailed Solution

The distance between the focus and pole of the mirror is called the focal length of the mirror.

Prove R = 2f 

Let F be the focus, P is the pole of the mirror R be the radius of curvature of the mirror and 'C' be its centre of curvature. 

Let a ray parallel to the principal axis striking the mirror at M. Then CM will be perpendicular mirror at M. Let θ be the angle of incidence and MD be the perpendicular from M on the principal axis.

Then from triangle MCP, tanθ=MDCDMDCP
When θ is small θ=MDR
From triangle MFD, tan2θ=MDFDMDFP
When 2θ is small, 2θ=MDf
2MDR=MDf2R=1f R=2f

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