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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