Editing Wave (section)

{{Short description|Repeated oscillation around equilibrium}}
{{About|waves in the scientific sense|waves on seas and lakes|Wind wave|the human hand gesture|Waving|other uses|Wave (disambiguation)|and|Wave motion (disambiguation)}}
[[File:2006-01-14 Surface waves.jpg|thumb|Surface waves in water showing water ripples]]

In [[physics]], [[mathematics]], [[engineering]], and related fields, a '''wave''' is a propagating dynamic disturbance (change from [[List of types of equilibrium|equilibrium]]) of one or more [[quantities]]. ''[[Periodic wave]]s'' oscillate repeatedly about an equilibrium (resting) value at some [[frequency]]. When the entire [[waveform]] moves in one direction, it is said to be a '''travelling wave'''; by contrast, a pair of [[superposition principle|superimposed]] periodic waves traveling in opposite directions makes a ''[[standing wave]]''. In a standing wave, the amplitude of vibration has nulls at some positions where the wave amplitude appears smaller or even zero.

There are two types of waves that are most commonly studied in [[classical physics]]: [[mechanical wave]]s and [[electromagnetic wave]]s. In a mechanical wave, [[Stress (mechanics)|stress]] and [[Strain (mechanics)|strain]] fields oscillate about a [[mechanical equilibrium]]. A mechanical wave is a local [[deformation (physics)|deformation (strain)]] in some physical medium that propagates from particle to particle by creating local [[stress (mechanics)|stresses]] that cause strain in neighboring particles too. For example, [[sound]] waves are variations of the local [[Sound pressure|pressure]] and [[Particle velocity|particle motion]] that propagate through the medium. Other examples of mechanical waves are [[seismic wave]]s, [[gravity wave]]s, [[surface wave]]s and [[string vibration]]s. In an electromagnetic wave (such as light), coupling between the electric and magnetic fields sustains propagation of waves involving these fields according to [[Maxwell's equations]]. Electromagnetic waves can travel through a [[vacuum]] and through some [[dielectric]] media (at wavelengths where they are considered [[transparency and translucency|transparent]]). Electromagnetic waves, as determined by their frequencies (or [[wavelength]]s), have more specific designations including [[radio wave]]s, [[infrared radiation]], [[terahertz waves]], [[visible light]], [[ultraviolet radiation]], [[X-ray]]s and [[gamma ray]]s.

Other types of waves include [[gravitational wave]]s, which are disturbances in [[spacetime]] that propagate according to [[general relativity]]; [[heat equation|heat diffusion waves]]; [[plasma wave]]s that combine mechanical deformations and electromagnetic fields; [[reaction–diffusion system|reaction–diffusion waves]], such as in the [[Belousov–Zhabotinsky reaction]]; and many more. Mechanical and electromagnetic waves transfer [[energy]],<ref>{{Harv|Hall|1980| p=8}}</ref> [[momentum (physics)|momentum]], and [[information]], but they do not transfer particles in the medium. In mathematics and [[electronics]] waves are studied as [[signal]]s.<ref>Pragnan Chakravorty, "What Is a Signal? [Lecture Notes]",&nbsp;IEEE ''Signal Processing Magazine'', vol. 35, no. 5, pp. 175–177, Sept. 2018.&nbsp;{{doi|10.1109/MSP.2018.2832195}}</ref> On the other hand, some waves have [[Envelope (waves)|envelopes]] which do not move at all such as [[standing wave]]s (which are fundamental to music) and [[hydraulic jump]]s.

[[File:Santos E et al Neuroimage 2014 .gif|thumb|Example of biological waves expanding over the brain cortex, an example of [[Cortical spreading depression|spreading depolarizations]].<ref>{{Cite journal|last1=Santos|first1=Edgar|last2=Schöll|first2=Michael|last3=Sánchez-Porras|first3=Renán|last4=Dahlem|first4=Markus A.|last5=Silos|first5=Humberto|last6=Unterberg|first6=Andreas|last7=Dickhaus|first7=Hartmut|last8=Sakowitz|first8=Oliver W.|date=2014-10-01|title=Radial, spiral and reverberating waves of spreading depolarization occur in the gyrencephalic brain|journal=NeuroImage|volume=99|pages=244–255|doi=10.1016/j.neuroimage.2014.05.021|issn=1095-9572|pmid=24852458|s2cid=1347927}}</ref>]]

A physical wave [[field (physics)|field]] is almost always confined to some finite region of space, called its ''domain''. For example, the seismic waves generated by [[earthquakes]] are significant only in the interior and surface of the planet, so they can be ignored outside it. However, waves with infinite domain, that extend over the whole space, are commonly studied in mathematics, and are very valuable tools for understanding physical waves in finite domains.

A ''[[plane wave]]'' is an important mathematical idealization where the disturbance is identical along any (infinite) plane [[Normal (geometry)|normal]] to a specific direction of travel. Mathematically, the simplest wave is a ''[[sinusoidal plane wave]]'' in which at any point the field experiences [[simple harmonic motion]] at one frequency. In linear media, complicated waves can generally be decomposed as the sum of many sinusoidal plane waves having [[Angular spectrum method|different directions of propagation]] and/or [[Fourier transform|different frequencies]]. A plane wave is classified as a ''[[transverse wave]]'' if the field disturbance at each point is described by a vector perpendicular to the direction of propagation (also the direction of energy transfer); or ''[[longitudinal wave]]'' if those vectors are aligned with the propagation direction. Mechanical waves include both transverse and longitudinal waves; on the other hand electromagnetic plane waves are strictly transverse while sound waves in fluids (such as air) can only be longitudinal. That physical direction of an oscillating field relative to the propagation direction is also referred to as the wave's ''[[polarization (waves)|polarization]]'', which can be an important attribute.
{{Modern physics}}