First slide

The lens

Question

A convex lens has 9 cm focal length and a concave lens has – 18 cm focal length. The focal length of the combination in contact will be

Moderate

A

9 cm
B

-18 cm
C

-9 cm
D

18 cm

Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \Rightarrow \frac{1}{F} = \frac{1}{(+ 18)} \Rightarrow F = 18 cm$

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