What is the term for an inward-curved surface in optics?

The term for an inward-curved surface in optics is indeed "concave." A concave surface is characterized by its shape that curves inward, resembling the interior of a bowl. This type of surface is significant in optics, as it can converge light rays toward a focal point. In practical applications, concave lenses and mirrors are used to magnify images or to focus light.

Understanding concave surfaces is essential in lens design and optical systems, as they afford unique optical properties that are utilized in devices such as eyeglasses, cameras, and telescopes. The way light interacts with a concave surface fundamentally differs from how it interacts with surfaces that are convex or flat. For instance, convex surfaces curve outward and diverge light rays, while flat surfaces neither converge nor diverge light, maintaining a straight path. Aspheric surfaces are designed to reduce optical aberrations but do not specifically refer to the inward curvature characteristic of concave surfaces.