Waves undergo a phase change upon reflection due to the reversal of their direction of propagation.

When a wave is reflected, it experiences a phase change because its direction of travel is inverted. This property is fundamental to all types of waves, including light waves, sound waves, and surface waves on a body of water. The phase of a wave indicates its position within the cycle of oscillation. Therefore, when a wave is reflected, it is effectively ‘flipped’ or ‘reversed’, resulting in a change in phase.

To understand the phase change upon reflection, consider a wave interacting with a boundary. When a wave reaches a boundary—such as the end of a string or a mirror—it cannot simply stop. Instead, it reflects back along the path it originally traveled. This reflection involves a reversal in the wave’s direction of propagation, which is associated with a phase change.

For instance, let’s examine a wave traveling along a string. If the wave moves to the right and encounters a fixed end, it will be reflected back to the left. This change in direction corresponds to a phase shift of $180^\circ$, or $\pi$ radians. This phenomenon is often referred to as phase reversal.

In the case of light waves, the magnitude of the phase change upon reflection can depend on the characteristics of the reflecting surface. When light is reflected from a medium with a higher refractive index, a phase change of $180^\circ$ occurs. Conversely, if the light reflects off a medium with a lower refractive index, no phase change takes place.

Understanding phase changes upon reflection is essential in various fields of physics, including optics and wave mechanics. This concept also plays a critical role in the functioning of numerous technological devices, such as lasers and fiber-optic cables.