The refractive index of the material of a double convex lens is 1.5 and its focal length is $5\;cm$. If the ratii of curvature are equal , the value of the radius of curvature (in cm) is

Question

\[\begin {array} {1 1} (a)\;5.0 & \quad (b)\;6.5 \\ (c)\; 13.5 & \quad (d)\;15.5 \end {array}\]