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 is equal to:
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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 is equal to:

Options

G1 Physics · Accepted Answer

Step 1: A parallel beam through convex lens A converges to its back focus 20 cm to the right. Step 2: Concave lens B gives a parallel output if the rays converge toward its virtual object-side arrangement at 5 cm to its right. Step 3: Separation d must equal f_A + f_B = 20 + (−5) = 15 cm. Final answer: (B) 15 cm.