Question

A plano convex lens is made of material having refractive index $1.5$. The radius of curvature of curved surface is $60$cm. The focal length of the lens is ___________cm.

• A. $-60$
• B. $120$
• C. $60$
• D. $-120$

Answer 1

The correct option is B $120$

Radius of curvature of plane surface is infinity i.e. $R_1=\infty$
Radius of curvature of convex surface $R_2=-60cm$
Given : $\mu =1.5$
Using $\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
Or $\frac{1}{f}=(1.5-1)(\frac{1}{\infty }-\frac{1}{-60})$
$⟹f=120cm$
So, correct answer is option B.