The focal length of a lens depends on

Correct answer: The radii of curvature of its surfaces and the refractive index of the lens with respect to surrounding medium.

Explanation:

The focal length (f) of a lens is given by the Lens Maker’s formula:

$\frac{1}{f} = \left(\frac{n_2}{n_1} - 1\right)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$

where:

$n_2$ = refractive index of lens material,

$n_1$ = refractive index of surrounding medium,

$R_1$ and $R_2$ = radii of curvature of the two surfaces.

Hence, the focal length depends on both the radii of curvature and the refractive index of the lens with respect to the surrounding medium.