LYCOS RETRIEVER 
Lens: Surfaces
built 621 days ago
Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.
Source:
If both surfaces are convex (curved outward from the body of the lens), the lens is thicker at its center than its edges. A lens with one surface convex and one concave is called meniscus. A lens with one flat surface is called plano-concave or plano-convex, depending on the nature of the other surface.
Source:
If the lens is convex-concave (a meniscus lens), whether it is converging or diverging depends on the relative curvatures of the two surfaces. If the curvatures are equal, then the beam is neither converged nor diverged.
Source:
MORE ABOUT
Surfaces