Question

Double convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?

• A. 11 cm
• B. 22 cm
• C. 7 cm
• D. 6 cm

Answer 1

The correct option is B 22 cm

Lens maker's formula,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
Here, f = 20 cm, $\mu$ = 1.55, $R_1$ = R, $R_2$ = -R
$\frac{1}{20}=(1.55-1)(\frac{1}{R}-\frac{1}{(-R)})or\frac{1}{20}=0.55\times \frac{2}{R}$
$\Rightarrow R=1.1\times 20=22cm$