The radii of curvature of a glass lens of focal length 7.5 cm are 5 cm and 15 cm. Then refractive index of glass is

$$\begin{gathered} \mathrm{f}=7.5\mathrm{cm},{\mathrm{R}}_1=5\mathrm{cm},{\mathrm{R}}_2=-15\mathrm{cm} \\ \mathrm{\mu }=? \end{gathered}$$
 

$\begin{array}{l}\frac{1}{\mathrm{f}}=(\mathrm{\mu }-1)\left(\frac{1}{{\mathrm{R}}_1}-\frac{1}{{\mathrm{R}}_2}\right)\\ \frac{1}{7.5}=(\mathrm{\mu }-1)\left(\frac{1}{5}-\frac{1}{15}\right)\\ \Rightarrow \frac{1}{7.5}=(\mathrm{\mu }-1)\left(\frac{3+1}{15}\right)\\ \Rightarrow \frac{1}{7.5}=(\mathrm{\mu }-1)\left(\frac{4}{15}\right)\\ \Rightarrow \mathrm{\mu }-1=\frac{1}{7.5}\times \frac{15}{4}\\ \Rightarrow \mathrm{\mu }-1=\frac{1}{2}\\ \Rightarrow \mathrm{\mu }=\frac{1}{2}+1\\ \Rightarrow \mathrm{\mu }=\frac{3}{2}\end{array}$