Phase change (waves)

Illustration of phase shift. The horizontal axis represents an angle (phase) that is increasing with time.

A phase change or phase shift is a change in the phase of a wave.

Optics

Light waves change phase by 180 degrees when they reflect from the surface of a medium with higher refractive index than that of the medium in which they are travelling.^[1] A light wave travelling in air that is reflected by a glass barrier will undergo a 180 degree phase change, while light travelling in glass will not undergo a phase change if it is reflected by a boundary with air. For this reason, optical boundaries are normally specified as an ordered pair (air-glass, glass-air); indicating which material the light is moving out of, and in to, respectively.

The phase changes that take place upon reflection play an important part in thin film interference.

Sound waves

Regarding longitudinal sound waves, when propagation occurs from a solid boundary to an air boundary the wave that is reflected back into the solid will experience a phase reversal.

References