What does the normal line represent in relation to a mirror?

The normal line is a critical concept in understanding how light interacts with reflective surfaces like mirrors. It is defined as a line that is drawn perpendicular to the surface of the mirror at the point where the incident ray strikes the mirror. Being perpendicular means it forms a 90-degree angle with the mirror's surface.

This is important because the behavior of light upon reflection is determined in relation to this normal line. Laws of reflection state that the angle of incidence (the angle between the incident ray and the normal line) is equal to the angle of reflection (the angle between the reflected ray and the normal line). Understanding the normal line helps clarify these angles and underpins many principles of optics and light behavior.

In this context, other choices do not accurately represent the normal line's role. For instance, a line at 45° to the mirror does not provide the necessary reference for angles of incidence or reflection. A line drawn parallel to the incident ray would also misrepresent the relationship needed for applying the reflection laws. Lastly, while the line showing the path of the reflected ray is a relevant concept, it does not define the normal line itself, which is solely about the perpendicularity to the mirror surface.