Radii of curvature of a convex lens are 20 cm and 30 cm and its focal length if 24 cm. Refractive index of the lens will be

1.33 1.5 1.71 None of these

1.5

As $\frac{1}{f}=(μ-1)(\frac{1}{R_1}-\frac{1}{R_1})$ $∴\frac{1}{24}=(μ-1)(\frac{1}{20}-\frac{1}{-30})=\frac{(μ-1)}{12}⇒μ=1.5$