What type of surface is characterized by an outward curvature?

The correct answer, convex, refers to a surface that curves outward, resembling the exterior of a sphere or balloon. In optical terms, lenses that have a convex surface are thicker at the center and taper towards the edges. This outward curvature helps to converge light rays that pass through the lens, making convex lenses essential in correcting farsightedness or for applications that require magnification.

In contrast, a concave surface curves inward, akin to an indented area. These surfaces diverge light rays, making them useful for situations like correcting nearsightedness.

A flat surface has no curvature, meaning it is entirely level across its area, and a planar surface also denotes a flat area. While these types of surfaces are significant in different contexts, they do not exhibit the characteristic outward curvature that defines a convex surface. Understanding these different curvature types is crucial for an optician, as it directly relates to the function and purpose of various optical devices.