Question

The radius of curvature for a convex lens is 40 cm. If its refractive index is 1.5, the focal length will be __________.

• A. 40 cm
• B. 20 cm
• C. 80 cm
• D. 60 cm

Answer 1

The correct option is A 40 cm

Given:
Radius of curvature $R_1,R_2=40cm$
Refractive index $n=1.5$
We know by lens maker's formula,

$\frac{1}{f}=(n-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$=(1.5-1)(\frac{1}{40}-\frac{1}{-40})=0.5\left(\frac{1}{20}\right)=40cm$