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 D
Given:
Radii of curvature = $R = 20\text{ cm} = 0.20\text{ m}$
Refractive index = $\mu = 1.5$
Lensmaker formula (power):
$P = (\mu-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
For a symmetric biconcave lens with equal radii,
$P = -\frac{2(\mu-1)}{R}$
Substitute values:
$P = -\frac{2(1.5-1)}{0.20} = -\frac{2\times 0.5}{0.20} = -\frac{1}{0.20} = -5\ \text{D}$
The power of the lens is $-5\ \text{dioptre}$.