A biconvex lens has radii of curvature, 20 cm each. If the refractive index of the material of the lens is 1.5, the power of the lens is :

1. infinity

2. +2D

3. +20 D

4. +5D

Doubtly Asked 30 Oct 2026 NEET UG 2022 Practice test neet Save WhatsApp Report

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Answer: 4. +5D

Use the lens maker's formula: \(\dfrac1f=(\mu-1)\left(\dfrac1{R_1}-\dfrac1{R_2}\right)\).

For a biconvex lens \(R_1=+20\) cm, \(R_2=-20\) cm:

\(\dfrac1f=0.5\left(\dfrac1{20}+\dfrac1{20}\right)=\dfrac{1}{20}\ \text{cm}^{-1}\Rightarrow f=20\ \text{cm}=0.2\ \text{m}\)

Power \(P=1/f=+5\) D. Option (4).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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