First slide

The lens

Question

A concave lens of focal length 20 cm placed in contact with a plane mirror acts as a

Moderate

A

Convex mirror of focal length 10 cm
B

Concave mirror of focal length 40 cm
C

Concave mirror of focal length 60 cm
D

Concave mirror of focal length 10 cm

Solution

$\frac{1}{F} = \frac{2}{f} + \frac{1}{f_{m}} . Here f_{m} = \infty, hence F = \frac{f}{2} = 10 cm$

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