A biconvex lens has a focal length (2 ÷ 3) times the radius of curvature of either surface. The refractive index of the lens material is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 1.75
Use the lens maker’s formula. For a biconvex lens with equal radii, \( R_1 = +R \) and \( R_2 = -R \) by the sign convention.
\( \displaystyle \frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) = (\mu - 1)\,\frac{2}{R} \)
Putting \( f = \frac{2}{3}R \):
\( \displaystyle \frac{3}{2R} = \frac{2(\mu - 1)}{R} \)
\( \displaystyle \mu - 1 = \frac{3}{4} \)
\( \displaystyle \mu = 1.75 \)
Note: A similar question was also asked in CUET (UG) 2025 on 02-06-2025 (Slot 2).