An equi-convex lens (R=10cm) has focal length equal to the focal length of a concave mirror (in magnitude). The refractive index of glass is . The radius of curvature of the concave mirror is

$$\begin{gathered} \text{ Focal length of convex lens }\frac{1}{f}(n-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right) \\ \begin{array}{l}\left.R_1=10\mathrm{cm},{\mathrm{R}}_2=-10\mathrm{cm},\mathrm{n}=1.5\text{ (for glass }\right)\\ \frac{1}{\mathrm{f}}(1.5-1)\left(\frac{1}{10}-\frac{1}{(-10)}\right)=0.5\left(\frac{2}{10}\right)\mathrm{f}=\frac{10}{2\times 0.5}=10\mathrm{cm}\end{array} \end{gathered}$$
$\therefore$Focal length of concave mirror = -10cm
$\therefore$ Radius of curvature $=2\times (-10)=-20\mathrm{cm}$