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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