The lens is used in optical instrument to focus and disperse the light beam. Most lenses are spherical lenses, including plano convex lens, plano concave lens, convex-concave lens, etc.These lenses have two surfaces polished, each face could be convex or concave or flat. The line joining the centers of the spheres making the lens surfaces is called axis of the lens. Typically the lens axis passes the center of the lens, as the way they are produced.

The lenses are classified by the curvature of the two optical faces. A lens is double convex if two faces are convex. If both surfaces have the same radii of curvature, the lens is equiconvex. If one of the faces is planner, the lens is plano convex lens or plano concave lens depending on the curvature of the other faces. A lens with one convex and one concave face called meniscus lens.

If the lens is plano convex lens or bi convex lens, the collimated beam of light passing the lens converges to a spot behind the lens, we call this lens a positive or converging lens. The distance from the lens to the spot is the focal length.