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Chromatic Aberrations

Two plano con...

Question

Two plano convex lenses of same material and of focal lengths 20 cm & 5 cm should be arranged such that the combination is free from chromatic aberration. Then effective focal length of the combination will be:

A

20 cm

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B

4 cm

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C

12.5 cm

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D

8 cm

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Solution

The correct option is C 12.5 cm
The effective focal length of the lens is,

$\frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{f_{1} + f_{2}}{2 f_{1} f_{2}}$

Substituting the respective values, the effective focal length obtained is, $f = 12.5$

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