What is the normal line in relation to a mirror?

The normal line is defined as a line that is perpendicular to the surface of the mirror at the point where the incident light ray strikes it. When light hits the surface of a mirror, it interacts with that specific point, and the normal line drawn at that point helps to establish the angles of incidence and reflection, which are crucial in the study of how light behaves when it encounters reflective surfaces.

This perpendicular relationship is essential because it allows us to apply the law of reflection, which states that the angle of incidence is equal to the angle of reflection. Without the normal line, it would be more challenging to understand and visualize these angles and their respective relationships to the light's behavior.

The other options do not accurately describe the function of the normal line. For instance, a line parallel to the mirror would not provide the necessary context for understanding light reflection, while a line showing the path of light does not specifically relate to the angles involved during reflection. Additionally, although the angle of reflection is determined using the normal line, it does not itself represent the normal line. Therefore, the correct characterization of the normal line as one that is perpendicular to the point of incidence on the mirror helps clarify its importance in optics.