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

1. f₁ + f₂
2. −f₁ + f₂
3. 2f₁ + f₂
4. −2f₁ + f₂

Correct answer: 2f₁ + f₂

Solution

For the light to return to infinity, the system must act as an equivalent plane mirror. This happens when the distance between the lens and the mirror equals the sum of twice the focal length of the concave mirror (2f₁) and the focal length of the convex lens (f₂).

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