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.

optical phase changes have been utilized, for instance, in the coating of lenses and optical filters.

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.