JEE Main 2026 — Jan 21, Shift 1 — Physics solution

JEE Main2026Jan 21, Shift 1Physics

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.

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Solution

A Keplerian beam expander uses two converging lenses sharing a common focal point. The angular magnification (and the beam-diameter ratio for collimated beams) equals $f_{2} / f_{1}$ .

$\frac{f _{2}}{f _{1}} = \frac{d _{o u t}}{d _{in}} = \frac{14}{2} = 7 \Rightarrow f_{2} = 7 \times 40 = 280$ mm.

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