Reflection phase change

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

Optics

In wave optics, a phase change of shift can occur upon reflection (as with thin film interference). When light passes from a medium of higher refractive index to a medium of lower refractive index, it changes phase by 180 degrees.

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.