If one line is normal to another, then it is at right angles to it.

In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.

  • How a normal appears in a geometric drawing depends on the circumstances:
    • When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
    • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
    • When light hits a curved surface, the normal line is drawn straight up from the point where the light hits the surface.
    • If light travels directly through the centre of a sphere, the normal line also passes through the centre of the sphere.
Remember that: