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# Two plano convex lenses of same material and of focal lengths 20 cm & 5 cm should be arranged such that the combination is free from chromatic aberration. Then effective focal length of the combination will be:20 cm4 cm12.5 cm8 cm

A
20 cm
B
12.5 cm
C
8 cm
D
4 cm
Solution
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#### The effective focal length of the lens is, 1f=1f1+1f2−f1+f22f1f2Substituting the respective values, the effective focal length obtained is, f=12.5

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