The power of a biconvex lens is 20 dioptre and the radius of curvature of each surface is 5 cm. Find the refractive index of the material of lens

Given, plower of lens, P=20 D Power of lens (in dioptre)
$\begin{array}{l}\mathrm{P}=\frac{100}{\text{ focal length }\mathrm{f}\left(\text{ in cm }\right)}\\ \Rightarrow \mathrm{f}=\frac{100}{20}=5\mathrm{cm}\end{array}$
From lens Maker’s formula,
$\frac{1}{\mathrm{f}}=(\mathrm{n}-1)\left(\frac{1}{{\mathrm{R}}_1}-\frac{1}{{\mathrm{R}}_2}\right)$
For biconvexlens , ${\mathrm{R}}_1=+\mathrm{R}$
and ${\mathrm{R}}_2=-\mathrm{R}$
$\begin{array}{l}\Rightarrow \frac{1}{\mathrm{f}}=(\mathrm{n}-1)\left(\frac{1}{\mathrm{R}}+\frac{1}{\mathrm{R}}\right)=(\mathrm{n}-1)\left(\frac{2}{\mathrm{R}}\right)\\ \frac{1}{5}=(\mathrm{n}-1)\left(\frac{2}{5}\right)[\because \text{ given },\mathrm{R}=5\mathrm{cm}]\\ 2\mathrm{n}-2=1\Rightarrow \mathrm{n}=3/2\end{array}$