convex

Turn a shiny spoon over and look at the back. That surface, which bulges toward you, is convex! The word describes any surface that curves outward, like the outside of a ball or a dome. You will often hear it used in optics; a convex mirror, like the one on the passenger side of a car, makes objects look smaller and provides a wider field of view. In geometry, a convex polygon is a shape where all the corners "point" outward, meaning there are no indentations or inward-bending angles.

The word "convex" describes a shape or surface that curves outward, much like the outside of a circle or the surface of a sphere. To visualize this, think of the gentle swell of a hill, the outer curve of an eyeball, or the surface of a magnifying glass. Unlike its opposite, "concave" (which hollows inward like a cave), a convex surface bulges toward the observer.

In technical fields, "convex" has very precise meanings. In optics, a convex lens is thicker in the middle than at the edges and is used to focus light, making it essential for magnifying glasses and spectacles that treat farsightedness. In mathematics, a "convex set" is a collection of points where a straight line segment connecting any two points within the set stays entirely inside the boundaries. Similarly, a convex polygon is a shape with no "dents"; no interior angle is greater than 180 degrees.

You will encounter "convex" in various contexts: geography (to describe rounded landforms), architecture (for domes and arched ceilings), and even computer graphics. Whenever you see a curve that bows outward with a sense of gentle expansion, the word you are looking for is "convex."