Question

A plano-convex lens is made of refractive index of $1.6 .$ The radius of curvature of the curved surface is $60\, cm$. The focal length of the lens is

• A. 400 cm
• B. 200 cm
• C. 100 cm
• D. 50 cm

Answer 1

Answer: (C)

100 cm

Answer 2

Here, $\frac{1}{f}=(\mu-1)\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]$ $\frac{1}{f}=(1.6-1)\left[\frac{1}{60}-\frac{1}{\infty}\right]=\frac{1}{100}$ $\Rightarrow f=100\, cm$