JEE Main 2026 — Apr 4, Shift 2 — Physics solution

JEE Main2026Apr 4, Shift 2Physics

A convex lens is made from glass material having refractive index of 1.4 with same radius of curvature on both sides. The ratio of its focal length and radius of curvature is ______.

A
0.5
B
2.5
C
0.8
D
1.25

Solution

Lensmaker's equation, with $R_{1} = R$ and $R_{2} = - R$ (equiconvex):

$\frac{1}{f} = (n - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}}) = (1.4 - 1) (\frac{1}{R} - \frac{1}{- R}) = 0.4 \cdot \frac{2}{R} = \frac{0.8}{R}$ .

$∴ \frac{f}{R} = \frac{1}{0.8} = 1.25$ .

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