A concave shape is one that curves inward, resembling the interior of a bowl or a cave. In geometry, a concave polygon has at least one interior angle greater than 180 degrees, causing it to "cave in" at that angle. This contrasts with a convex shape, where all angles are less than 180 degrees and the shape bulges outward. Concave mirrors and lenses are common examples of concave surfaces. A concave mirror reflects light inward to a focal point, making it useful in applications like telescopes and makeup mirrors. Similarly, a concave lens diverges light rays, which is helpful in correcting vision problems like nearsightedness.