Editing Wave vector

{{Short description|Vector describing a wave; often its propagation direction}}
{{Use American English|date=January 2019}}

In [[physics]], a '''wave vector''' (or '''wavevector''') is a [[vector (geometric)|vector]] used in describing a [[wave]], with a typical unit being cycle per metre. It has a [[Euclidean vector|magnitude and direction]]. Its magnitude is the [[wavenumber]] of the wave (inversely proportional to the [[wavelength]]), and its direction is perpendicular to the [[wavefront]].  In isotropic media, this is also the direction of [[wave propagation]].

A closely related vector is the '''angular wave vector''' (or '''angular wavevector'''), with a typical unit being radian per metre.  The wave vector and angular wave vector are related by a fixed constant of proportionality, 2{{pi}}&nbsp;radians per cycle.

It is common in several fields of [[physics]] to refer to the angular wave vector simply as the ''wave vector'', in contrast to, for example, [[crystallography]].<ref>Physics example: {{cite book| url=https://books.google.com/books?id=c60mCxGRMR8C&pg=PA288 | title= Handbook of Physics| author= Harris, Benenson, Stöcker|page=288| isbn=978-0-387-95269-7| year=2002}}</ref><ref>Crystallography example: {{cite book| url=https://books.google.com/books?id=xjIGV_hPiysC&pg=PA259 | title=Modern Crystallography |author=Vaĭnshteĭn| page=259| isbn=978-3-540-56558-1| year=1994}}</ref>  It is also common to use the symbol {{mvar|'''k'''}} for whichever is in use.

In the context of [[special relativity]], a ''[[wave four-vector]]'' can be defined, combining the (angular) wave vector and (angular) frequency.

==Definition==
{{See also|Traveling wave}}
[[File:Sine wavelength.svg|thumb|right|Wavelength of a [[sine wave]], {{mvar|λ}}, can be measured between any two consecutive points with the same [[phase (waves)|phase]], such as between adjacent crests, or troughs, or adjacent [[zero crossing]]s with the same direction of transit, as shown.]] 

The terms ''wave vector'' and ''angular wave vector'' have distinct meanings.  Here, the wave vector is denoted by <math> \tilde{\boldsymbol{\nu}}  </math> and the wavenumber by <math>\tilde{\nu} = \left| \tilde{\boldsymbol{\nu}} \right|</math>.  The angular wave vector is denoted by {{math|'''k'''}} and the angular wavenumber by {{math|1=''k'' = {{abs|'''k'''}}}}.  These are related by <math>\mathbf k = 2\pi \tilde{\boldsymbol{\nu}}</math>.

A sinusoidal [[traveling wave]] follows the equation
:<math>\psi(\mathbf{r},t) = A \cos (\mathbf{k} \cdot \mathbf{r} - \omega t + \varphi) ,</math>
where:
* {{math|'''r'''}} is position,
* {{mvar|t}} is time,
* {{mvar|&psi;}} is a function of {{math|'''r'''}} and {{mvar|t}} describing the disturbance describing the wave (for example, for an [[ocean wave]], {{mvar|&psi;}} would be the excess height of the water, or for a [[sound wave]], {{mvar|&psi;}} would be the excess [[air pressure]]).
* {{mvar|A}} is the [[amplitude]] of the wave (the peak magnitude of the oscillation),
* {{mvar|&phi;}} is a [[phase offset]],
* {{mvar|&omega;}} is the (temporal) [[angular frequency]] of the wave, describing how many radians it traverses per unit of time, and related to the [[Period (physics)|period]] {{mvar|T}} by the equation <math>\omega= \tfrac{2\pi}{T},</math>
* {{math|'''k'''}} is the angular wave vector of the wave, describing how many radians it traverses per unit of distance, and related to the [[wavelength]] by the equation <math>|\mathbf{k}|= \tfrac{2\pi}{\lambda}.</math>

The equivalent equation using the wave vector and frequency is<ref>{{cite book |url=https://books.google.com/books?id=xjIGV_hPiysC&pg=PA259|title= Modern Crystallography| page=259 |isbn=978-3-540-56558-1 |last=Vaĭnshteĭn|first=Boris Konstantinovich |year=1994}}</ref>
:<math> \psi \left( \mathbf{r}, t \right) = A \cos \left(2\pi \left( \tilde{\boldsymbol{\nu}} \cdot {\mathbf r} - f t \right) + \varphi \right) ,</math>
where:
* <math> f </math> is the frequency
* <math> \tilde{\boldsymbol{\nu}}  </math> is the wave vector

==Direction of the wave vector==
{{Main|Group velocity}}
The direction in which the wave vector points must be distinguished from the "direction of [[wave propagation]]". The "direction of wave propagation" is the direction of a wave's energy flow, and the direction that a small [[wave packet]] will move, i.e. the direction of the [[group velocity]]. For light waves in vacuum, this is also the direction of the [[Poynting vector]]. On the other hand, the wave vector points in the direction of [[phase velocity]]. In other words, the wave vector points in the [[surface normal|normal direction]] to the [[Wave front|surfaces of constant phase]], also called [[wavefronts]].

In a [[attenuation|lossless]] [[isotropy|isotropic medium]] such as air, any gas, any liquid, [[amorphous solids]] (such as [[glass]]), and [[cubic crystal]]s, the direction of the wavevector is the same as the direction of wave propagation. If the medium is anisotropic, the wave vector in general points in directions other than that of the wave propagation. The wave vector is always perpendicular to surfaces of constant phase.

For example, when a wave travels through an [[anisotropy|anisotropic medium]], such as [[crystal optics|light waves through an asymmetric crystal]] or sound waves through a [[sedimentary rock]], the wave vector may not point exactly in the direction of wave propagation.<ref name=fowles>{{cite book|last=Fowles|first=Grant|title=Introduction to modern optics|year=1968|publisher=Holt, Rinehart, and Winston|page=177}}</ref><ref name=pollard>"This effect has been explained by Musgrave (1959) who has shown that the energy of an elastic wave in an anisotropic medium will not, in general, travel along the same path as the normal to the plane wavefront ...", ''Sound waves in solids'' by Pollard, 1977. [https://books.google.com/books?id=EOUNAQAAIAAJ link]</ref>

==In solid-state physics==
{{Main|Bloch's theorem}}
In [[solid-state physics]], the "wavevector" (also called '''k-vector''') of an [[electron]] or [[electron hole|hole]] in a [[crystal]] is the wavevector of its [[quantum mechanics|quantum-mechanical]] [[wavefunction]]. These electron waves are not ordinary [[sinusoidal]] waves, but they do have a kind of ''[[Envelope (waves)|envelope function]]'' which is sinusoidal, and the wavevector is defined via that envelope wave, usually using the "physics definition". See [[Bloch's theorem]] for further details.<ref>{{cite book |author=Donald H. Menzel |title=Fundamental Formulas of Physics, Volume 2 |chapter-url=https://books.google.com/books?id=-miofZvrH2sC&pg=PA624 |page=624 |chapter=§10.5 Bloch wave |isbn=978-0486605968 |year=1960 |edition=Reprint of Prentice-Hall 1955 2nd |publisher=Courier-Dover }}</ref>

==In special relativity==
A moving wave surface in special relativity may be regarded as a hypersurface (a 3D subspace) in spacetime, formed by all the events passed by the wave surface.  A wavetrain (denoted by some variable {{mvar|X}}) can be regarded as a one-parameter family of such hypersurfaces in spacetime.  This variable {{mvar|X}} is a scalar function of position in spacetime.  The derivative of this scalar is a vector that characterizes the wave, the four-wavevector.<ref>{{cite book |author=Wolfgang Rindler |title=Introduction to Special Relativity |pages=[https://archive.org/details/introductiontosp0000rind/page/60 60–65] |section=§24 Wave motion |isbn=978-0-19-853952-0 |year=1991 |edition=2nd |publisher=Oxford Science Publications |url=https://archive.org/details/introductiontosp0000rind/page/60 }}</ref>

The four-wavevector is a wave [[four-vector]] that is defined, in [[Minkowski space|Minkowski coordinates]], as:

:<math>K^\mu = \left(\frac{\omega}{c}, \vec{k}\right) = \left(\frac{\omega}{c}, \frac{\omega}{v_p}\hat{n}\right) = \left(\frac{2 \pi}{cT}, \frac{2 \pi \hat{n}}{\lambda}\right) \,</math>

where the angular frequency <math>\tfrac{\omega}{c}</math> is the temporal component, and the wavenumber vector <math>\vec{k}</math> is the spatial component.

Alternately, the wavenumber {{mvar|k}} can be written as the angular frequency {{mvar|&omega;}} divided by the [[phase velocity|phase-velocity]] {{mvar|v{{sub|p}}}}, or in terms of inverse period {{mvar|T}} and inverse wavelength {{mvar|&lambda;}}.

When written out explicitly its [[Covariance and contravariance of vectors|contravariant]] and [[Covariance and contravariance of vectors|covariant]] forms are: 
:<math>\begin{align}
  K^\mu &= \left(\frac{\omega}{c}, k_x, k_y, k_z \right)\, \\[4pt]
  K_\mu &= \left(\frac{\omega}{c}, -k_x, -k_y, -k_z \right)
\end{align}</math>

In general, the [[Lorentz scalar]] magnitude of the wave four-vector is:

:<math>K^\mu K_\mu = \left(\frac{\omega}{c}\right)^2 - k_x^2 - k_y^2 - k_z^2 = \left(\frac{\omega_o}{c}\right)^2 = \left(\frac{m_o c}{\hbar}\right)^2</math>

The four-wavevector is [[Causal structure#Tangent vectors|null]] for [[Massless particle|massless]] (photonic) particles, where the rest mass <math>m_o = 0</math>

An example of a null four-wavevector would be a beam of coherent, [[monochromatic]] light, which has phase-velocity <math>v_p = c</math>

:<math>K^\mu = \left(\frac{\omega}{c}, \vec{k}\right) = \left(\frac{\omega}{c}, \frac{\omega}{c}\hat{n}\right) = \frac{\omega}{c}\left(1, \hat{n}\right) \,</math> {for light-like/null}

which would have the following relation between the frequency and the magnitude of the spatial part of the four-wavevector:

:<math>K^\mu K_\mu = \left(\frac{\omega}{c}\right)^2 - k_x^2 - k_y^2 - k_z^2 = 0</math> {for light-like/null}

The four-wavevector is related to the [[four-momentum]] as follows:
:<math>P^\mu = \left(\frac{E}{c}, \vec{p}\right) = \hbar K^\mu = \hbar\left(\frac{\omega}{c}, \vec{k}\right) </math>

The four-wavevector is related to the [[four-frequency]] as follows:
:<math>K^\mu = \left(\frac{\omega}{c}, \vec{k}\right) = \left(\frac{2 \pi}{c}\right)N^\mu = \left(\frac{2 \pi}{c}\right)\left(\nu, \nu \vec{n}\right)</math>

The four-wavevector is related to the [[four-velocity]] as follows:
:<math>K^\mu = \left(\frac{\omega}{c}, \vec{k}\right) = \left(\frac{\omega_o}{c^2}\right)U^\mu = \left(\frac{\omega_o}{c^2}\right) \gamma \left(c, \vec{u}\right)</math>

===Lorentz transformation===
Taking the [[Lorentz transformation]] of the four-wavevector is one way to derive the [[relativistic Doppler effect]].  The Lorentz matrix is defined as
:<math>\Lambda = \begin{pmatrix}
           \gamma & -\beta \gamma & \ 0 \ & \ 0 \ \\
    -\beta \gamma &        \gamma & 0     & 0 \\
                0 &             0 & 1     & 0 \\
                0 &             0 & 0     & 1
 \end{pmatrix}</math>

In the situation where light is being emitted by a fast moving source and one would like to know the frequency of light detected in an earth (lab) frame, we would apply the Lorentz transformation as follows.  Note that the source is in a frame {{math|''S''<sup>s</sup>}} and earth is in the observing frame, {{math|''S''<sup>obs</sup>}}.
Applying the Lorentz transformation to the wave vector
:<math>k^{\mu}_s = \Lambda^\mu_\nu k^\nu_{\mathrm{obs}} </math>

and choosing just to look at the <math>\mu = 0</math> component results in
:<math>\begin{align}
             k^{0}_s &= \Lambda^0_0 k^0_{\mathrm{obs}} + \Lambda^0_1 k^1_{\mathrm{obs}} + \Lambda^0_2 k^2_{\mathrm{obs}} + \Lambda^0_3 k^3_{\mathrm{obs}} \\[3pt]
  \frac{\omega_s}{c} &= \gamma \frac{\omega_{\mathrm{obs}}}{c} - \beta \gamma k^1_{\mathrm{obs}} \\
                     &= \gamma \frac{\omega_{\mathrm{obs}}}{c} - \beta \gamma \frac{\omega_{\mathrm{obs}}}{c} \cos \theta.
\end{align}</math>

where <math>\cos \theta </math> is the direction cosine of <math>k^1</math> with respect to <math>k^0, k^1 = k^0 \cos \theta. </math>

So
:{|cellpadding="2" style="border:2px solid #ccccff"
|<math>\frac{\omega_{\mathrm{obs}}}{\omega_s} = \frac{1}{\gamma (1 - \beta \cos \theta)} </math>
|}

====Source moving away (redshift)====
As an example, to apply this to a situation where the source is moving directly away from the observer (<math>\theta=\pi</math>), this becomes:
:<math>\frac{\omega_{\mathrm{obs}}}{\omega_s} = \frac{1}{\gamma (1 + \beta)} = \frac{\sqrt{1-\beta^2}}{1+\beta} = \frac{\sqrt{(1+\beta)(1-\beta)}}{1+\beta} = \frac{\sqrt{1-\beta}}{\sqrt{1+\beta}} </math>

====Source moving towards (blueshift)====
To apply this to a situation where the source is moving straight towards the observer ({{math|1=''&theta;'' = 0}}), this becomes:

:<math>\frac{\omega_{\mathrm{obs}}}{\omega_s} = \frac{1}{\gamma (1 - \beta)} = \frac{\sqrt{1-\beta^2}}{1-\beta} = \frac{\sqrt{(1+\beta)(1-\beta)}}{1-\beta} = \frac{\sqrt{1+\beta}}{\sqrt{1-\beta}} </math>

====Source moving tangentially (transverse Doppler effect)====
To apply this to a situation where the source is moving transversely with respect to the observer ({{math|1=''&theta;'' = ''&pi;''/2}}), this becomes:
:<math>\frac{\omega_{\mathrm{obs}}}{\omega_s} = \frac{1}{\gamma (1 - 0)} = \frac{1}{\gamma} </math>

==See also==
* [[Plane-wave expansion]]
* [[Plane of incidence]]

==References==
{{notelist}}
{{Reflist}}

==Further reading==
*{{cite book | author=Brau, Charles A. | title=Modern Problems in Classical Electrodynamics | publisher=Oxford University Press | year=2004 | isbn=978-0-19-514665-3}}

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[[Category:Wave mechanics]]
[[Category:Vector physical quantities]]