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NEET 2012 (AIPMT, Prelims) · Combination of Lenses

Combination of Lenses question from NEET 2012 (AIPMT, Prelims)

Q74NEET 2012 · AIPMT · PRELIMSMedium

Combination of Lenses

A concave mirror of focal length $f$ 1 is placed at a distance of d from a convex lens of focal length $f$ 2. A beam of light coming from infinity and falling on this convex lens $-$ concave mirror combination returns to infinity. The distance d must equal

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Answer: (c)

The lens focuses the parallel beam at $f_2$ behind it.
To return to infinity the rays must retrace their path, i.e. hit the mirror normally: the focus must be at the mirror's centre of curvature, $2f_1$ from the mirror.
$d = f_2 + 2f_1$
So the answer is (c) $2f_1 + f_2$ .

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