A plano convex lens of focal length 120 cm has a convex surface of radius of curvature 50 cm.  The refractive index of the lens is

_{$\frac{1}{f}=\left(\mu -1\right)\frac{1}{R}$}

_{$\Rightarrow \frac{1}{12\cancel{0}}=\left(\mu -1\right)\frac{1}{5\cancel{0}}$}

_{$\Rightarrow \frac{5}{12}=\mu -1$}                                                                                             

_{$\mu =\frac{5}{12}+1=\frac{17}{12}=1.4cm$}