What does the angle of incidence measure?

The angle of incidence specifically measures the angle formed between the incident ray (the incoming ray of light) and the normal line, which is an imaginary line that is perpendicular to the surface at the point of incidence. This concept is fundamental in understanding how light interacts with surfaces, especially when looking at reflections and how angles affect that process.

The normal is essential because it provides a reference point to define the angles involved in reflection and refraction. For example, when light strikes a mirror, the angle of incidence is critical for applying the law of reflection, which states that the angle of incidence is equal to the angle of reflection.

Understanding this concept helps clarify other aspects of light behavior, such as when light enters a new medium, where the angle of incidence can impact refraction, but that would involve different terminology and details not directly related to the angle of incidence itself.