Reflection phase change
A phase change sometimes occurs when a wave is reflected.[1][2] Such reflections occur for many types of wave, including light waves, sound waves, and waves on strings.
Optics
Light waves change phase by 180° 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° 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.
"Phase" here is the phase of the electric field oscillations, not the magnetic field oscillations.[3] Also, this is referring to near-normal incidence—for p-polarized light reflecting off glass at glancing angle, beyond the Brewster angle, the phase change is 0°.
The phase changes that take place upon reflection play an important part in thin film interference.
Sound waves
Sound waves in a solid experience a phase reversal (a 180° change) when they reflect from a boundary with air.[2] Sound waves in air do not experience a phase change when they reflect from a solid, but they do exhibit a 180° change when reflecting from a region with lower acoustic impedance. An example of this is when a sound wave in a hollow tube encounters the open end of the tube. The phase change on reflection is important in the physics of wind instruments.
Strings
A wave on a string experiences a 180° phase change when it reflects from a point where the string is fixed.[2] Reflections from the free end of a string exhibit no phase change. The phase change when reflecting from a fixed point contributes to the formation of standing waves on strings, which produce the sound from stringed instruments.
References
- ^ a b Nave, C.R. "Reflection Phase Change". Hyperphysics. Georgia State University. Retrieved 2016-03-28.
- ^ a b c Nave, C.R. "Reflection of Sound". Hyperphysics. Georgia State University. Retrieved 2016-03-28.
- ^ Byrnes, Steven J. (2016). "Multilayer optical calculations". arXiv:1603.02720. Appendix A