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A plano convex lens has focal length f =...

A plano convex lens has focal length `f = 20 cm`. If its plane surface is silvered, then new focal length will be

A

`20 cm`

B

40 cm

C

30 cm

D

10 cm

Text Solution

Verified by Experts

The correct Answer is:
D
`1/(f) = (2)/(f_(L)) + (1)/(f_(M))`
For a plane mirror, `f_(M) = oo`
`:. 1/f = 2/(f_(L))` or `f = (f_(L))/(2) = (20 cm)/(2) = 10 cm`
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