What is an imaginary line drawn perpendicular to a boundary called?

An imaginary line drawn perpendicular to a boundary is referred to as a normal. In the context of physics and particularly optics, the normal line is crucial when discussing the reflection and refraction of light. When light interacts with a surface, understanding the normal is important because it helps define angles of incidence and reflection. The angle between the incoming light ray and the normal is known as the angle of incidence, while the angle between the reflected ray and the normal is known as the angle of reflection. This relationship is fundamental to the laws of reflection, where the angle of incidence equals the angle of reflection.

The other terms do not accurately describe this concept; an axis generally refers to a line that defines the center of rotation or symmetry in geometry, while radius and diameter pertain specifically to circles, describing measurements related to the circle itself rather than indicating a perpendicular line to a boundary. Therefore, the identification of the normal as the line perpendicular to a boundary is essential in understanding geometrical optics.