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JEE Main 2026 (21 Jan, Shift 1) · Combination of Lenses

Combination of Lenses question from JEE Main 2026 (21 Jan, Shift 1)

Q7JEE Main 2026 · 21 JAN · SHIFT 1Medium

Combination of Lenses

A collimated beam of light of diameter 2 mm is propagating along $x$ -axis. The beam is required to be expanded in a collimated beam of diameter 14 mm using a system of two convex lenses. If first lens has focal length 40 mm, then the focal length of second lens is ______ mm.

Numerical answer type. Work it out, then open the answer.

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Answer: 280

Beam expander: lenses separated by $f_1 + f_2$ (common focus).
Beam width ratio $= \frac{f_2}{f_1} = \frac{14}{2} = 7$
$f_2 = 7 \times 40 = 280\text{ mm}$

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