The imaginary line perpendicular to the surface of a medium where a wave strikes it is called the normal.

Here's why it's important:

* Angle of Incidence and Reflection: The angle of incidence (the angle between the incoming wave and the normal) is equal to the angle of reflection (the angle between the reflected wave and the normal). This is a fundamental law of reflection.

* Refraction: When a wave passes from one medium to another, it bends. The angle of refraction (the angle between the refracted wave and the normal) is related to the angle of incidence and the properties of the two media.

* Wave Interference: The concept of the normal is crucial when considering how waves interfere with each other, especially at boundaries between different media.

So, the normal is a crucial reference line for understanding how waves interact with surfaces.