A wave is turned back when it meets the boundary of the medium in which it is traveling. The wave is said to have undergone

The wave is said to have undergone reflection.

Explanation:

When a wave traveling through a medium meets the boundary of that medium, it gets bounced back and returns into the medium in which it is traveling. This behavior of the wave that causes a change in the direction of the wave is known as reflection. Examples of reflection include the reflection of sound waves, light waves, water waves, etc.

According to the law of reflection, the angle at which the waves approach the barrier (or an interface between two mediums) equals the angle at which they reflect off the barrier (or an interface between two mediums).

Refraction of waves is referred to the change in the direction of waves or bending of the path of the waves when they pass from one medium to another which is accompanied by a change in wavelength and speed of the waves.

Diffraction of waves is referred to the change in the direction of waves when they move around a barrier in their path or pass through an opening. Interference occurs when two waves traveling through the same medium superpose to form a wave having the same or lower or greater amplitude.