If in a plano-convex lens, the radius of the convex surface is 10 cm and the focal length is 30 cm. Then the refractive index of the material will be:

1.33

$\frac{1}{f} = (\mu -1)(\frac{1}{R_1} - \frac{1}{R_2})$

$\Rightarrow \frac{1}{30} = (\mu -1)(\frac{1}{10} - \frac{1}{\infty}) = \frac{(\mu - 1)}{10}$

$\Rightarrow \mu -1 = \frac{1}{3}$

$\Rightarrow \mu = 1.33$