Question

# Two plano convex lenses of focal lengthsf1 andf2 set for minimising spherical aberration are kept apart by 2 cm. If the equivalent focal length of combination is 10 cm, the values of f1 and f2 are :

A
18cm,16cm
B
18cm,20cm
C
20cm,22cm
D
14cm,20cm
Solution
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#### d=f2−f1⇒f2−f1=2 ⇒f2=2+f1feq=f1f2f1+f2−d=f1f2f1+f2−2=10(f1)(2×f1)(2f1)=102+f1=20f1=18cmf2=2+f1=20cm

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