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Question: A plano-convex (\(\int = 20 \mathrm {~cm}\)) is silvered at plane surface. The focal length will be ...

A plano-convex (∫=20 cm\int = 20 \mathrm {~cm}) is silvered at plane surface. The focal length will be :

A

20 cm

B

40 cm

C

30 cm

D

10 cm

Answer

10 cm

Explanation

Solution

:

For a plane mirror, ,

∴1f=2fL\therefore \frac { 1 } { \mathrm { f } } = \frac { 2 } { \mathrm { f } _ { \mathrm { L } } }

or f=fL2=20 cm2=10 cm\mathrm { f } = \frac { \mathrm { f } _ { \mathrm { L } } } { 2 } = \frac { 20 \mathrm {~cm} } { 2 } = 10 \mathrm {~cm}