First slide

The lens

Question

The plane surface of a plano-convex lens of focal length f is silvered. It will behave as

Moderate

A

Plane mirror
B

Convex mirror of focal length 2f
C

Concave mirror of focal length $\frac{f}{2}$
D

None of the above

Solution

Focal length of effective lens
$\frac{- 1}{F} = \frac{2}{f_{l}} + \frac{1}{f_{m}} = \frac{2}{f_{l}} + \frac{1}{\infty} \Rightarrow F = \frac{- f_{l}}{2}$

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