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

1.33              1.5  1.71 None of these

1.5

$\text{Focal length of lens in air is }\frac{1}{f_a} = (\mu_g - 1)(\frac{1}{R_1} - \frac{1}{R_2})$ $ \frac{1}{24} = (\mu_g - 1)(\frac{1}{20} + \frac{1}{30}) =(\mu_g - 1)\frac{1}{12} $ $ \mu_g - 1 = 0.5$ $\Rightarrow \mu_g = 1.5$