First slide

The lens

Question

Two thin lenses, one of focal length + 60 cm and the other of focal length – 20 cm are put in contact. The combined focal length is

Moderate

A

$+ 15 cm$
B

-15 cm
C

+30 cm
D

-30 cm

Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{1}{60} + \frac{1}{(- 20)} \Rightarrow F = - 30$ cm

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