The radii of curvature of two surfaces of a convex lens are 10 cm and 20 cm, respectively. The focal length of the lens in air is 20 cm. The refractive index of the material of the lens is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4/3
The lens maker’s formula links the focal length to the refractive index and the radii. For a double convex lens, with the Cartesian sign convention the first surface has \( R_1 = +10 \) cm and the second \( R_2 = -20 \) cm.
\( \displaystyle \frac{1}{f} = (n - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
\( \displaystyle \frac{1}{20} = (n - 1)\left(\frac{1}{10} + \frac{1}{20}\right) = (n - 1)\frac{3}{20} \)
\( \displaystyle n - 1 = \frac{1}{3} \)
\( \displaystyle n = \frac{4}{3} \)
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Power of a Lens – …
