Two plano-convex lenses of focal lengths f and 2f are coaxially separated so that the spherical aberration is minimized. The effective focal length of the combination is :

3f2

f

3f4

4f3

A

3f4

B

3f2

C

f

D

4f3

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Solution

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d=f2−f1=2f−f=f

feq=f1f2f1+f2−d=2f×ff+2f−f=f

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