Question

what is the focal length of plano convex lens kept in air with radii R=60cm. take refractive index of lens n=1.5.

Answer 1

use the formula,
$\frac{1}{f}=(\mu-1)[\frac{1}{R_1}-\frac{1}{R_2}]$

For Plano convex lens ,
R₁ = ∞ , and R₂ = -R

1/f = (μ-1)[ 1/∞ - 1/-R ]
f = R/(μ - 1)

now, put R = 60cm , μ = n = 1.5
then, f = 60/(1.5-1) = 120 cm