NEET 2012 Physics question
A concave mirror of focal length is placed at a distance of from a convex lens of focal length . A beam of light coming from infinity and falling on this convex lens-concave mirror combination returns to infinity. The distance '' must equal
Answer: (4)
Solution
A parallel beam from infinity is first focused by the convex lens at its focus, a distance from the lens. For the light finally to return to infinity as a parallel beam (retracing its path), the rays after reflection must again diverge from this point, so this point must be imaged on itself by the concave
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