# convex

A convex lens focuses parallel rays of light to a point.

Convex means curved outward, or having positive curvature, as in certain mirrors and
lenses; the word comes from the Latin *convexus* for "vaulted." A **biconvex
lens** has two convex surfaces, not necessarily with the same radii
of curvature.

A figure, such as a polygon or a polyhedron, is said to be convex if every line segment that joins two interior points remains inside the figure. Similarly, a set is convex if it contains all the line segments connecting any pair of its points. Compare with concave.