What does the normal refer to in wave theory?

In wave theory, the normal refers to a line that is drawn at a right angle to the surface at the point of incidence, which is where a wave meets a boundary. This concept is crucial when analyzing how waves behave at interfaces, such as when they reflect or refract.

When a wave reaches a boundary between two different media, the angle of incidence is measured from the normal. Understanding the normal helps in applying Snell's Law for refraction, as well as the law of reflection, where the angle of reflection equals the angle of incidence. This alignment allows for clear predictions about the wave's behavior based on its interaction with surfaces.

Using the normal is essential for visualizing and calculating wave behaviors, which makes it a foundational concept in understanding wave interactions in various physical contexts.