A convex lens A of focal length 20cm and a concave lens B of focal length 5cm are kept along the same axis with the distance d between them. If a prallel beam of light falling on A leaves B as a parallel beam, then the distance d in cm will be

15

25

30

50

A

15

B

25

C

50

D

30

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Solution

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The correct option is A15 d=f1−f2 By using the formula 1f1+1f1−df1f2=1F For the emergent beam to parallel P=0,F=α ⇒120−15+d100=0 ⇒d=15

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