Q 12-09-080JEE MainJEE Main 2025 (29 Jan, Shift 2)Easy
Two identical symmetric double convex lenses of focal length $f$ are cut into two equal parts, $L_1, L_2$ by the $AB$ plane and $L_3, L_4$ by the $XY$ plane, as shown in the figure respectively. The ratio of focal lengths of lenses $L_1$ and $L_3$ is
Answer: (B) $1:2$
- Cut along $AB$ (the principal axis): each half keeps both curved surfaces, so $L_1$ still has focal length $f$ (only its aperture is smaller).
- Cut along $XY$ (the plane of the lens): each half is a plano-convex lens with half the power, so $L_3$ has focal length $2f$.
$f_1 : f_3 = f : 2f = 1 : 2$
Solution by Sreeraj P, M.Sc Physics