Question

What is the focal length of double concave lens kept in air with two spherical surfaces of radii $R_1=30cm$ and $R_2=60cm$. Take refractive index of lens with respect to surrounding as n = 1.5.

Answer 1

Given,
$R_1=-30cm$, $R_2=60cm$, $n_e=-1.5$

Using lens maker's formula,
$\frac{1}{f}=(n_e-1)(\frac{1}{R_1}-\frac{1}{R_2}$ )
$\frac{1}{f}=(1.5-1)(\frac{1}{-30}-\frac{1}{60}$ )
$\frac{1}{f}=(1.5-1)\left(\frac{-3}{60}\right)$
$\frac{1}{f}=\left(0.5\right)\left(\frac{-1}{20}\right)$
$\frac{1}{f}=\frac{-5}{200}$
$f=-40cm$