Generally, light traveling through lenses is bent twice: once when it enters the lens and once when it leaves. In the case of a thin lens, it is possible to neglect this double bending.
A lens with both surfaces convex (more generally, a lens that is thicker in the middle than on the edges) is a converging lens, while a lens with both surfaces concave (more generally, a lens that is thicker at the edges than at the middle) is a diverging lens. Focal length and the other quantities describing mirrors apply to lenses as well. Additionally, lenses obey the Lens maker's equation:
where & are the radii of curvature of the first and second surfaces of the lens and n is the index of refraction of the lens material.
