What does the normal line indicate in optics?

The normal line is a key concept in optics, particularly when discussing the behavior of light as it interacts with surfaces, such as mirrors or boundaries between different media. It is defined as a line that is perpendicular to the surface at the point where the incident light ray strikes.

Understanding why this is the correct choice hinges on the role that the normal line plays in the laws of reflection and refraction. The angle of incidence, which is the angle between the incident ray and the normal line, is crucial for determining how the light will reflect off the surface. According to the law of reflection, the angle of reflection is equal to the angle of incidence, both of which are measured relative to the normal line.

This clarification emphasizes that the normal line itself is not a representation of the incident ray's direction or any other processes related to reflection, but strictly serves as a reference point for measuring angles related to light's interaction with surfaces. Thus, the normal line's perpendicular nature is fundamental to understanding optics, particularly in calculating and predicting light behavior.