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# 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 be15253050

A
15
B
25
C
50
D
30
Solution
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#### The correct option is A15d=f1−f2By using the formula1f1+1f1−df1f2=1FFor the emergent beam to parallelP=0,F=α⇒120−15+d100=0⇒ d=15

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