Two plano-convex lenses of same material and of focal lengths f and 2f are coaxially separated by a suitable distace to minimize chromatic aberration. The equivalent focal length of the combination is

f

3f/2

3f/4

4f/3

A

3f/4

B

3f/2

C

f

D

4f/3

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Solution

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d=f2+f12=2f+f2=3f2

feq=f×2f−3f2+2f+f=2f23f2=4f3

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