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 :

Step-by-step solution

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