Question

A double convex lens is made of a glass of refractive index $1.55$, with both faces of the same radius of curvature. Find the radius of curvature required, if the focal length is $20$cm.

Answer 1

Double convex lenses are to be manufactured with both a glass of same refractive index and same radius of curvature.
Given $\mu =1.55$ and one glass radius be $R_1=R$ while other be $R_2=-R$
Focal length $f=20cm$
Now
$$\frac{1}{f}=(\mu -1)[\frac{1}{R_1}-\frac{1}{R_2}]=(\mu -1)[\frac{1}{R}+\frac{1}{R}]=\frac{1.1}{R}$$
So, radius of curvature R=22cm