Question

The focal length of a biconvex lens is 20 cm and its refractive index is 1.5. If the radii of curvatures of two surfaces of lens are in the ratio 1 : 2, then the larger radius of curvature is :

• A. 10 cm
• B. 15 cm
• C. 20 cm
• D. 30 cm

Answer 1

Answer: (D)

30 cm

From lens maker's formula, $\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

$\frac{1}{20}=(1.5-1)(\frac{1}{R}-\frac{1}{(-2R)})$

$\frac{1}{20}=\frac{1}{2}\times \frac{3}{2R}$

$R=15cm$

$\therefore$ length of radius =$\left(2R\right)=2\times 15cm=30cm$