What is the term for the point of intersection of the axis of a lens or mirror with its surface?

The term that describes the point of intersection of the axis of a lens or mirror with its surface is called the vertex. In the context of optical devices such as lenses and mirrors, the vertex is particularly important because it serves as a reference point from which various measurements, such as focal length and optical axis alignment, can be made. The vertex is the surface point that the principal axis passes through, functioning as a critical location for understanding how light interacts with the optical element.

In contrast, while the apex might refer to the highest point in a context such as a geometrical figure, it is not specifically used to describe a lens or mirror's point of intersection. The focus refers to the point where light rays converge after passing through a lens or reflecting off a mirror. The center could refer to the geometric center of a lens or mirror, but it does not denote the specific intersection point on the surface with the axis. Each of these terms serves different functions and meanings in optics, making it essential to distinguish between them.