A small astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. If an object is situated 200 cm away from the objective, the lens must be separated by a distance:
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Question

A small astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. If an object is situated 200 cm away from the objective, the lens must be separated by a distance:

Options

G1 Physics · Accepted Answer

Step 1: Objective has fₒ = 40 cm and object distance u = −200 cm. Step 2: Lens formula 1/v − 1/u = 1/fₒ gives 1/v = 1/40 − 1/200 = 1/50, so v = +50 cm. Step 3: For final image at infinity, eyepiece front focus coincides with the objective image. Hence objective–eyepiece separation L = v + fₑ = 50 + 4 = 54 cm. Final answer: (C) 54.0 cm.