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 a distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then the distance d in cm will be :

Question

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 a distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then the distance d in cm will be :, 50, 15, 25, 30

MARKS App · Accepted Answer

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 = 15 c m From figure it is clear that separation between lenses, d = 20 − 5 = 15 cm

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