What is the term for a line that is perpendicular to a surface?

The term used to describe a line that is perpendicular to a surface is "normal." In optics and physics, when discussing reflections, refractions, or interactions of different forms of waves with surfaces, it is crucial to identify the normal line. This line helps define angles of incidence and reflection, which are measured relative to the normal. The normal line is essential in calculations involving the behavior of light or waves at interfaces.

The other terms do not accurately represent a line perpendicular to a surface. An axis usually refers to a central line around which a body rotates or a coordinate system. An incident ray describes a ray of light striking a surface, and while related to the normal, it is not defined as perpendicular. A tangent, on the other hand, refers to a line that touches a curve at one point and does not cross it, which is in contrast to the normal line.