What does the term 'normal' refer to in the context of reflective surfaces?

In the context of reflective surfaces, the term 'normal' refers specifically to a line that is perpendicular to the surface of the mirror or any reflective medium. This perpendicular line is significant in optics because it is used as a reference point for measuring angles of incidence and reflection as defined by the law of reflection. According to this law, when a beam of light strikes a reflective surface, the angle at which it hits the surface (the angle of incidence) is equal to the angle at which it reflects away (the angle of reflection). The normal line serves as the baseline from which these angles are measured, with both angles being measured from the normal to the incoming and outgoing light rays, respectively.

The other options, while they reference concepts related to optics, do not accurately define 'normal' in this context.