Question

The Radius of curvature of two surfaces of a convex lens are 10 cm and 20 cm respectively. The refrective index of glass is 1.5 then determine focal length of the lens.

Answer 1

We have,

$\begin{array}{c}\frac{1}{f}=\left(\frac{n_2}{n_1}-1\right)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)\\ \Rightarrow \frac{1}{f}=\left(1.5-1\right)\left(\frac{1}{10}-\frac{1}{20}\right)\end{array}$

$\begin{array}{c}\Rightarrow \frac{1}{f}=\frac{0.5}{10\times 10}\times \frac{1}{2}\\ \Rightarrow \frac{1}{f}=\frac{5}{200}\\ \Rightarrow \frac{1}{f}=40cm\end{array}$