First slide

The lens

Question

Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact convex. The effective power of the combination is

Moderate

A

45 dioptres
B

9 dioptres
C

$\frac{1}{9}$ dioptre
D

6 dioptres

Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{1}{20} + \frac{1}{25} \Rightarrow F = \frac{100}{9} cm = \frac{1}{9} metre$

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