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

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answer is B.

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Detailed Solution

P=1f1+1f2=df1f20=120−15−d20−5d100=15−120=4−120=320or d=15 cm

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