The radii of curvature of the two surfaces of a concave lens are 20 cm each. The refractive index of the material of the lens is 1.5. The power of the lens would be
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( -5\,\mathrm{D} \)
A concave (biconcave) lens has both surfaces curving inward. With light travelling from the left, the first surface has \( R_1 = -20\,\mathrm{cm} \) and the second \( R_2 = +20\,\mathrm{cm} \). Use the lens maker's formula and then \( P = 1/f \).
\( \displaystyle \frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) = 0.5\left(-\frac{1}{20} - \frac{1}{20}\right) = -\frac{1}{20} \)
\( \displaystyle f = -20\,\mathrm{cm} = -0.2\,\mathrm{m} \)
\( \displaystyle P = \frac{1}{f} = \frac{1}{-0.2} = -5\,\mathrm{D} \)