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Question

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
  1. 15
  2. 25
  3. 30
  4. 50

A
15
B
25
C
50
D
30
Solution
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The correct option is A 15
d=f1f2
By using the formula
1f1+1f1df1f2=1F
For the emergent beam to parallel
P=0,F=α
12015+d100=0
d=15

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