Spherical Lenses

The focal length of a lens can be calculated using the lensmaker's formula: where: is the refractive index of the lens material, and are the radii of curvature of the two surfaces of the lens. For a convex lens, both surfaces have radii of curvature of 40 cm, so: Note that is negative because the second surface of the lens is curved in the opposite direction. Substitute these values into the lensmaker’s formula: Thus, the focal length of the lens is 40 cm.