Question

A point object in air is in front of the curved surface of a planoconvex lens. The radius of curvature of the curved surface is $30\text{cm}$ and the refractive index of the lens material is $1.5$, then the focal length of the lens $\text{(in cm)}$ is

Answer 1

Given : $\mu =1.5;R_{\text{curved}}=30\text{cm}$
Using, Lens-maker formula
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
For plano-convex lens
$R_1\to \infty \text{then}{R}_2=-R$
$\therefore f=\frac{R}{\mu -1}=\frac{30}{1.5-1}=60\text{cm}$

Hence, $60$ is the correct answer.