A concave mirror of focal length f₁ is placed at a distance d from a convex lens of focal length f₂. A beam coming from infinity and falling on this convex lens–concave mirror combination returns to infinity. The distance d must be equal to:
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Question

A concave mirror of focal length f₁ is placed at a distance d from a convex lens of focal length f₂. A beam coming from infinity and falling on this convex lens–concave mirror combination returns to infinity. The distance d must be equal to:

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G1 Physics · Accepted Answer

Step 1: Parallel rays first converge at the back focal point of the convex lens, distance f₂ behind it. Step 2: For the concave mirror to return the rays along the same path, that focal point must coincide with the centre of curvature of the mirror, which lies 2f₁ in front of it. Step 3: Therefore lens–mirror separation d = f₂ + 2f₁ (if geometry uses the indicated positions). Final answer: Source key (C). Original answer options and diagram need confirmation.