Why do waves experience a phase change on reflection?

Waves experience a phase change on reflection due to the reversal of their direction of propagation.

When a wave is reflected, it undergoes a phase change because the direction of its propagation is reversed. This is a fundamental property of waves, whether they are light waves, sound waves, or waves in a pond. The phase of a wave describes its position in the cycle of oscillation. When a wave is reflected, it is essentially 'flipped' or 'reversed', which results in a phase change.

The phase change on reflection can be understood by considering a wave hitting a boundary. When a wave encounters a boundary, such as the end of a string or a mirror, it cannot simply stop. Instead, it is reflected back along the path it came from. This reflection involves a reversal of the wave's direction of propagation, which corresponds to a phase change.

For example, consider a wave on a string. If the wave is moving to the right and hits a fixed end, it will be reflected back to the left. This change in direction corresponds to a phase change of 180 degrees, or π radians. This is often referred to as a phase reversal.

In the case of light waves, the phase change upon reflection depends on the nature of the reflecting surface. If light is reflected from a medium of higher refractive index, a phase change of 180 degrees occurs. However, if the reflection occurs from a medium of lower refractive index, there is no phase change.

Understanding the phase change on reflection is crucial in many areas of physics, including optics and wave mechanics. It is also fundamental to the operation of many technological devices, such as lasers and fibre-optic cables.