A bulb is located on a wall. Its image is to be obtained on a parallel wall with the help of a convex lens. If the distance between parallel walls is d, then the length of the lens placed in between the walls is:
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Question

A bulb is located on a wall. Its image is to be obtained on a parallel wall with the help of a convex lens. If the distance between parallel walls is d, then the length of the lens placed in between the walls is:

Options

G1 Physics · Accepted Answer

Step 1: For a real object-image pair separated by d = u_abs + v, lens equation gives 1/f = 1/u_abs + 1/v. Step 2: Product u_abs v is at most (d/2)² by AM–GM. Step 3: f = u_abs v/d ≤ d/4, with equality for equal object/image distances. Final answer: (D); mathematically the lens focal length must satisfy f ≤ d/4, so check the exact option wording.