Why is there a phase change during wave reflection?

A phase change during wave reflection occurs due to the reversal of direction of the wave upon hitting a boundary.

When a wave encounters a boundary, such as the end of a string or a change in medium, it can't simply disappear. Instead, it reflects back into the medium from which it came. This reflection involves a change in direction, which is equivalent to a phase change. The phase of a wave describes its position in the cycle of oscillation. When a wave reflects, it effectively turns around, which is equivalent to advancing or retarding by half a cycle, or 180 degrees. This is why we observe a phase change during wave reflection.

The specifics of the phase change depend on the nature of the boundary. If the wave is reflecting off a boundary that is fixed or denser than the medium the wave is travelling in, the reflected wave will undergo a phase change of 180 degrees, or a 'phase flip'. This means that the peaks become troughs and the troughs become peaks. This is often referred to as an 'inverted' reflection.

On the other hand, if the wave is reflecting off a boundary that is free or less dense than the medium the wave is travelling in, the reflected wave will not undergo a phase change. The peaks will still be peaks and the troughs will still be troughs after reflection. This is often referred to as an 'upright' reflection.

In summary, the phase change during wave reflection is a consequence of the wave's change in direction upon hitting a boundary. The specifics of the phase change, whether it's a phase flip or no phase change at all, depend on the nature of the boundary. Understanding this concept is crucial for understanding wave behaviour in various physical contexts, from light reflecting off mirrors to sound echoing in a room.