Question

An achromatic doublet is a converging lens of equivalent focal length . If the dispersive powers of the material of the lenses are in the ratio . The focal lengths of the two lenses are:

A

B

C

D

Hard
Solution
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Correct option is B)

, Now, from question:
For an achromatic combination, 

Also, the effective focal length of the combination will be,

Solving these simultaneously and substituting the values, 
the obtained focal lengths are:

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