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

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Correct answer: B. 15

(1)/(F) = 1 f _ 1 + 1 f _ 2 - d f _ 1 f _ 2 ∴ (1)/(∞) = (1)/(20) + (1)/(- 5) - (d)/(20(- 5)) ⇒ 0 = (1)/(20) - (1)/(5) + (d)/(100) ∴ (d)/(100) = (1)/(5) - (1)/(20) = (20 - 5)/(100) ∴ d = 15cm From figure it is clear that separation between lenses, d = 20 - 5 = 15cm

A convex lens A of focal length 20cm and a concave lens B of focal...

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