Double convex lenses are to be manufacture from a grass of refractive index 1.55 , with both a glass of refractive index 1.55 with both faces of same radius of curvature . What is the radius of curvature required if local length of lens is to be 20 cm ?

Question

$\begin{array}{1 1} 11\;cm \\ 22\;cm \\ 10\;cm \\ 20\;cm \end{array}$