Physics Q41 – JEE-MAIN 2026

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 _______.

Clue 1 of 3: Lens Type and Properties

Identify the type of lens and the sign conventions for its radii of curvature.

🧠 Full Solution Path

Step 1: Identify Lens Type and Radii of Curvature✦ Active

The lens is a convex lens with the same radius of curvature on both sides, which means it is a biconvex lens. For a biconvex lens, the first surface has a radius of curvature $R_{1} = R$ (positive, as light enters from the left, the first surface is convex) and the second surface has a radius of curvature $R_{2} = - R$ (negative, as the second surface is also convex but its center of curvature is on the left).

Step 2: Apply Lens Maker's Formula○ Expand

The Lens Maker's formula is given by:

\frac{1}{f} = (n - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})

Substitute the given values: refractive index $n = 1.4$ , $R_{1} = R$ , and $R_{2} = - R$ .

\frac{1}{f} = (1.4 - 1) (\frac{1}{R} - \frac{1}{- R}) = (0.4) (\frac{1}{R} + \frac{1}{R})

Step 3: Calculate the Ratio

f / R

○ Expand

Simplify the expression:

\frac{1}{f} = (0.4) (\frac{2}{R}) = \frac{0.8}{R}

Rearrange the equation to find the ratio $f / R$ :

\frac{f}{R} = \frac{1}{0.8} = \frac{10}{8} = 1.25

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Physics Question 41 – JEE-MAIN 2026

Clue 1 of 3: Lens Type and Properties

🧠 Full Solution Path