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Question

Two lenses of focal lengths $+ 10$ cm and $- 15$ cm when put in contact behave like a convex lens. They will have zero longitudinal chromatic aberration if their dispersive powers are in the ratio

A

$+ 3 / 2$

B

$+ 2 / 3$

C

$- 3 / 2$

D

$- 1 / 2$

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Solution

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Correct option is B)

For achromatic aberration:

$\frac{w _{1}}{f _{1}} + \frac{w _{2}}{f _{2}} = 0$

$\frac{w _{1}}{w _{2}} = \frac{- f _{1}}{f _{2}}$

$= \frac{- 1 0}{( - 1 5 )}$

$= \frac{+ 2}{3}$

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