The reflection of waves is accompanied by a phase change, which occurs when waves reflect off a boundary and may experience a phase shift of $180$ degrees.

When a wave encounters a boundary—such as the interface between two different media—some of the wave is reflected back into the original medium. This reflection can lead to a phase change, which is a shift in the positions of the wave’s peaks and troughs. The extent of this phase change is influenced by the properties of the boundary. Specifically, when a wave is reflected from a boundary that is fixed or denser than the medium from which the wave is traveling, it experiences a phase shift of $180$ degrees, commonly referred to as a phase reversal. In this case, the wave is inverted, meaning that peaks become troughs and vice versa.

Conversely, if the wave reflects off a boundary that is less dense or free to move, no phase change occurs. In this scenario, the reflected wave retains the same phase as the incident wave. This situation is typical for waves on a string that is free to move at one end, as well as for light waves reflecting off a less dense medium.

Understanding the concept of phase change is essential for analyzing the behavior of waves in various contexts, including wave interference. When two waves meet, they superpose, or add together. If the waves are in phase (meaning their peaks and troughs align), they will interfere constructively, resulting in a wave with increased amplitude. In contrast, if the waves are out of phase (where the peak of one wave aligns with the trough of another), they will interfere destructively, leading to a wave with reduced amplitude or even complete cancellation. Thus, the phase change occurring during reflection can significantly influence the resulting wave pattern.

In summary, the reflection of waves can involve a phase change depending on the characteristics of the boundary. This phase change—whether it results in a phase reversal or no phase change at all—plays a crucial role in the behavior and interactions of waves.