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

Options

A. 20 cm
B. 5 cm
C. 10 cm
D. 25 cm

Answer

C

Explanation

When plane surface is silvered, light suffers refraction at convex surface, then reflection from plane surface, then reflection from plane surface and final refraction at the curved surface again as shown in Fig.
`:.` Power of such a lens,
`P = P_1 + P_2 + P_1 = 2 P_1` `(because P_2 = 0)`
=`(2)/(f)=(2)/(0.2)= 10 D`
New focal length, `f = (100)/(P)=(100)/(10) = 10 cm`.
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