All rays of light passing through a lens are refracted (bent) except those that pass directly through a point called the optical center. Lenses are classified according to the way in which they bend the rays of light entering them. Parallel rays of light passing through converging lenses are bent toward one another; these lenses are thicker at the center than at the edges. Examples are the double convex lens (both surfaces curved outward as in the simple magnifying glass), the plano-convex (one flat and one convex surface), and the concavo-convex (one surface concave, the other convex). Diverging lenses bend parallel rays away from one another; they are thicker at the edges than at the center. Examples are the double concave lens (both surfaces curved inward), the plano-concave (one surface flat, the other concave), and the convexo-concave (one surface convex, the other concave).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.