First slide

The lens

Question

Two similar plano-convex lenses are combined together in three different ways as shown in the adjoining figure. The ratio of the focal lengths in three cases will be

Moderate

A

2 : 2 : 1
B

1 : 1 : 1
C

1 : 2 : 2
D

2 : 1 : 1

Solution

In each case two plane-convex lens are placed close to each other, and
$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{1}{2 f} + \frac{1}{2 f} = \frac{1}{f} F = f$

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