The lens formula relates the focal length of a lens with the distance of object and image.
\(\frac{1}{f}\) = (μ - 1) \((\frac{1}{R_1} - \frac{1}{R_2})\)
where µ is the refractive index of the material of the lens; R1 and R2 are the radii of curvature of the two faces of the lens; f is the focal length of the lens.