Wave#Travelling waves

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Wave#Travelling waves
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{{About|waves in the scientific sense|waves on the surface of the ocean or lakes|Wind wave|other uses|Wave (disambiguation)}}(File:Rectified waves.png|thumb|Different types of wave with varying rectifications)In physics, a wave is a disturbance that transfers energy through matter or space, with little or no associated mass transport. Waves consist of oscillations or vibrations of a physical medium or a field, around relatively fixed locations. From the perspective of mathematics, waves, as functions of time and space, are a class of signals.Pragnan Chakravorty, "What Is a Signal? [Lecture Notes]," IEEE Signal Processing Magazine, vol. 35, no. 5, pp. 175-177, Sept. 2018. weblink are two main types of waves: mechanical and electromagnetic. Mechanical waves propagate through a physical matter, whose substance is being deformed. Restoring forces then reverse the deformation. For example, sound waves propagate via air molecules colliding with their neighbours. When the molecules collide, they also bounce away from each other (a restoring force). This keeps the molecules from continuing to travel in the direction of the wave. Electromagnetic waves do not require a medium. Instead, they consist of periodic oscillations of electrical and magnetic fields originally generated by charged particles, and can therefore travel through a vacuum. These types vary in wavelength, and include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.Waves are described by a wave equation which sets out how the disturbance proceeds over time. The mathematical form of this equation varies depending on the type of wave. Further, the behavior of particles in quantum mechanics are described by waves. In addition, gravitational waves also travel through space, which are a result of a vibration or movement in gravitational fields.A wave can be transverse, where a disturbance creates oscillations that are perpendicular to the propagation of energy transfer, or longitudinal: the oscillations are parallel to the direction of energy propagation. While mechanical waves can be both transverse and longitudinal, all electromagnetic waves are transverse in free space.

General features

File:2006-01-14 Surface waves.jpg|thumb|Surface waves in waterwater
A single, all-encompassing definition for the term wave is not straightforward. A vibration can be defined as a back-and-forth motion around a reference value. However, a vibration is not necessarily a wave. An attempt to define the necessary and sufficient characteristics that qualify a phenomenon as a wave results in a blurred line.The term wave is often intuitively understood as referring to a transport of spatial disturbances that are generally not accompanied by a motion of medium occupying this space as a whole. In a wave, the energy of a vibration is moving away from the source in the form of a disturbance within the surrounding medium {{Harv|Hall|1982| p=8}}. However, this motion is problematic for a standing wave (for example, a wave on a string), where energy is moving in both directions equally, or for electromagnetic (e.g., light) waves in a vacuum, where the concept of medium does not apply and interaction with a target is the key to wave detection and practical applications. There are water waves on the ocean surface; gamma waves and light waves emitted by the Sun; microwaves used in microwave ovens and in radar equipment; radio waves broadcast by radio stations; and sound waves generated by radio receivers, telephone handsets and living creatures (as voices), to mention only a few wave phenomena.It may appear that the description of waves is closely related to their physical origin for each specific instance of a wave process. For example, acoustics is distinguished from optics in that sound waves are related to a mechanical rather than an electromagnetic wave transfer caused by vibration. Concepts such as mass, momentum, inertia, or elasticity, become therefore crucial in describing acoustic (as distinct from optic) wave processes. This difference in origin introduces certain wave characteristics particular to the properties of the medium involved. For example, in the case of air: vortices, radiation pressure, shock waves etc.; in the case of solids: Rayleigh waves, dispersion; and so on.Other properties, however, although usually described in terms of origin, may be generalized to all waves. For such reasons, wave theory represents a particular branch of physics that is concerned with the properties of wave processes independently of their physical origin.BOOK, Modulated waves: theory and application,weblink Lev A. Ostrovsky, Alexander I. Potapov, yes, Johns Hopkins University Press, 0-8018-7325-8, 2001, For example, based on the mechanical origin of acoustic waves, a moving disturbance in space–time can exist if and only if the medium involved is neither infinitely stiff nor infinitely pliable. If all the parts making up a medium were rigidly bound, then they would all vibrate as one, with no delay in the transmission of the vibration and therefore no wave motion. On the other hand, if all the parts were independent, then there would not be any transmission of the vibration and again, no wave motion. Although the above statements are meaningless in the case of waves that do not require a medium, they reveal a characteristic that is relevant to all waves regardless of origin: within a wave, the phase of a vibration (that is, its position within the vibration cycle) is different for adjacent points in space because the vibration reaches these points at different times repeatedly .

Mathematical description of one-dimensional waves

Wave equation

Consider a traveling transverse wave (which may be a pulse) on a string (the medium). Consider the string to have a single spatial dimension. Consider this wave as traveling(File:Nonsinusoidal wavelength.svg|thumb|right|200 px|Wavelength λ, can be measured between any two corresponding points on a waveform)(File:Superpositionprinciple.gif|thumb|Animation of two waves, the green wave moves to the right while blue wave moves to the left, the net red wave amplitude at each point is the sum of the amplitudes of the individual waves. Note that f(x,t) + g(x,t) = u(x,t))
  • in the x direction in space. E.g., let the positive x direction be to the right, and the negative x direction be to the left.
  • with constant amplitude u
  • with constant velocity v, where v is
BOOK, Seismic waves and rays in elastic media,weblink 131 ff, Michael A. Slawinski, Wave equations, 0-08-043930-6, 2003, Elsevier, This wave can then be described by the two-dimensional functions
u(x,t) = F(x - v t) (waveform F traveling to the right) u(x,t) = G(x + v t) (waveform G traveling to the left)
or, more generally, by d'Alembert's formula:BOOK, Wave motion in elastic solids, Karl F Graaf, Reprint of Oxford 1975, Dover, 1991,weblink 13–14, 978-0-486-66745-4,
u(x,t) = F(x-vt) + G(x+vt). ,representing two component waveforms F and G traveling through the medium in opposite directions. A generalized representation of this wave can be obtainedFor an example derivation, see the steps leading up to eq. (17) in WEB,weblink Kinematic Derivation of the Wave Equation, Francis Redfern, Physics Journal,

frac{1}{v^2}frac{partial^2 u}{partial t^2}=frac{partial^2 u}{partial x^2}. ,General solutions are based upon Duhamel's principle.BOOK, Geometric wave equations, Jalal M. Ihsan Shatah, Michael Struwe,weblink The linear wave equation, 37 ff, 0-8218-2749-9, 2000, American Mathematical Society Bookstore,

Wave forms

File:Waveforms.svg|thumb|right|280 px|Sine, square, triangle and sawtooth waveforms.]]The form or shape of F in d'Alembert's formula involves the argument x − vt. Constant values of this argument correspond to constant values of F, and these constant values occur if x increases at the same rate that vt increases. That is, the wave shaped like the function F will move in the positive x-direction at velocity v (and G will propagate at the same speed in the negative x-direction).BOOK,weblink 128 ff, All you wanted to know about mathematics but were afraid to ask, Louis Lyons, 0-521-43601-X, Cambridge University Press, 1998, In the case of a periodic function F with period λ, that is, F(x + λ − vt) = F(x − vt), the periodicity of F in space means that a snapshot of the wave at a given time t finds the wave varying periodically in space with period λ (the wavelength of the wave). In a similar fashion, this periodicity of F implies a periodicity in time as well: F(x − v(t + T)) = F(x − vt) provided vT = λ, so an observation of the wave at a fixed location x finds the wave undulating periodically in time with period T = λ/v.BOOK, Introduction to Macromolecular Crystallography, Alexander McPherson,weblink 77, Waves and their properties, 0-470-18590-2, 2009, 2, Wiley,

Amplitude and modulation

(File:Amplitudemodulation.gif|thumb|Amplitude modulation can be achieved through f(x,t) = 1.00*sin(2*pi/0.10*(x-1.00*t)) and g(x,t) = 1.00*sin(2*pi/0.11*(x-1.00*t))only the resultant is visible to improve clarity of waveform.)(File:Wave packet.svg|right|thumb|Illustration of the envelope (the slowly varying red curve) of an amplitude-modulated wave. The fast varying blue curve is the carrier wave, which is being modulated.){{See also|Frequency modulation|Phase modulation}}The amplitude of a wave may be constant (in which case the wave is a c.w. or continuous wave), or may be modulated so as to vary with time and/or position. The outline of the variation in amplitude is called the envelope of the wave. Mathematically, the modulated wave can be written in the form:BOOK,weblink 9, FEW-cycle Laser Dynamics and Carrier-envelope Phase Detection, Christian Jirauschek, 3-86537-419-0, 2005, Cuvillier Verlag, BOOK, Oscillations and waves, Fritz Kurt Kneubühl,weblink 365, 1997, 3-540-62001-X, Springer, BOOK,weblink 33, Mark Lundstrom, 0-521-63134-3, 2000, Fundamentals of carrier transport, Cambridge University Press,
u(x,t) = A(x,t)sin (kx - omega t + phi) ,
where A(x, t) is the amplitude envelope of the wave, k is the wavenumber and phi is the phase. If the group velocity v_g (see below) is wavelength-independent, this equation can be simplified as:BOOK,weblink Chin-Lin Chen, Foundations for guided-wave optics, 363, §13.7.3 Pulse envelope in nondispersive media, 0-471-75687-3, 2006, Wiley,
u(x,t) = A(x - v_g t)sin (kx - omega t + phi) ,
showing that the envelope moves with the group velocity and retains its shape. Otherwise, in cases where the group velocity varies with wavelength, the pulse shape changes in a manner often described using an envelope equation.BOOK, Localized Waves, Localization and Wannier wave packets in photonic crystals, Stefano Longhi, Davide Janner, Hugo E. Hernández-Figueroa, Michel Zamboni-Rached, Erasmo Recami,weblink 329, 0-470-10885-1, 2008, Wiley-Interscience,

Phase velocity and group velocity

{{See also|Envelope (waves)#Phase and group velocity}}
missing image!
- Wave group.gif -
frame|The red square moves with the phase velocity, while the green circles propagate with the group velocity
There are two velocities that are associated with waves, the phase velocity and the group velocity. Phase velocity is the rate at which the phase of the wave propagates in space: any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity is given in terms of the wavelength {{mvar|λ}} (lambda) and period {{mvar|T}} as
v_mathrm{p} = frac{lambda}{T}.
missing image!
- Wave opposite-group-phase-velocity.gif -
frame|A wave with the group and phase velocities going in different directions
Group velocity is a property of waves that have a defined envelope, measuring propagation through space (i.e. phase velocity) of the overall shape of the waves' amplitudes—modulation or envelope of the wave.

Sine waves

{{duplication|section=y|dupe=Sinusoidal wave|dupe2=Frequency|date=July 2015}}File:Simple harmonic motion animation.gif|thumb|right|Sinusoidal waves correspond to simple harmonic motionsimple harmonic motionMathematically, the most basic wave is the (spatially) one-dimensional sine wave (also called harmonic wave or sinusoid) with an amplitude u described by the equation:
u(x,t)= A sin (kx- omega t + phi) ,
  • A is the maximum amplitude of the wave, maximum distance from the highest point of the disturbance in the medium (the crest) to the equilibrium point during one wave cycle. In the illustration to the right, this is the maximum vertical distance between the baseline and the wave.
  • x is the space coordinate
  • t is the time coordinate
  • k is the wavenumber
  • omega is the angular frequency
  • phi is the phase constant.
The units of the amplitude depend on the type of wave. Transverse mechanical waves (e.g., a wave on a string) have an amplitude expressed as a distance (e.g., meters), longitudinal mechanical waves (e.g., sound waves) use units of pressure (e.g., pascals), and electromagnetic waves (a form of transverse vacuum wave) express the amplitude in terms of its electric field (e.g., volts/meter).The wavelength lambda is the distance between two sequential crests or troughs (or other equivalent points), generally is measured in meters. A wavenumber k, the spatial frequency of the wave in radians per unit distance (typically per meter), can be associated with the wavelength by the relation
k = frac{2 pi}{lambda}. ,The period T is the time for one complete cycle of an oscillation of a wave. The frequency f is the number of periods per unit time (per second) and is typically measured in hertz denoted as Hz. These are related by:
f=frac{1}{T}. ,In other words, the frequency and period of a wave are reciprocals.The angular frequency omega represents the frequency in radians per second. It is related to the frequency or period by
omega = 2 pi f = frac{2 pi}{T}. ,The wavelength lambda of a sinusoidal waveform traveling at constant speed v is given by:BOOK, Understanding physics, David C. Cassidy, Gerald James Holton, Floyd James Rutherford,weblink 339 ff, 0-387-98756-8, 2002, Birkhäuser,
lambda = frac{v}{f},
where v is called the phase speed (magnitude of the phase velocity) of the wave and f is the wave's frequency.Wavelength can be a useful concept even if the wave is not periodic in space. For example, in an ocean wave approaching shore, the incoming wave undulates with a varying local wavelength that depends in part on the depth of the sea floor compared to the wave height. The analysis of the wave can be based upon comparison of the local wavelength with the local water depth.BOOK, op. cit., Paul R Pinet,weblink 242, 0-7637-5993-7, 2009, Although arbitrary wave shapes will propagate unchanged in lossless linear time-invariant systems, in the presence of dispersion the sine wave is the unique shape that will propagate unchanged but for phase and amplitude, making it easy to analyze.BOOK, Communication Systems and Techniques, Mischa Schwartz, William R. Bennett, Seymour Stein, yes, John Wiley and Sons, 1995, 978-0-7803-4715-1, 208,weblink
Due to the Kramers–Kronig relations, a linear medium with dispersion also exhibits loss, so the sine wave propagating in a dispersive medium is attenuated in certain frequency ranges that depend upon the medium.
See Eq. 5.10 and discussion in BOOK, A. G. G. M. Tielens, The physics and chemistry of the interstellar medium,weblink 119 ff, 0-521-82634-9, 2005, Cambridge University Press, ; Eq. 6.36 and associated discussion in BOOK, Introduction to solid-state theory, Otfried Madelung,weblink 261 ff, 3-540-60443-X, 1996, 3rd, Springer, ; and Eq. 3.5 in BOOK, F Mainardi, Transient waves in linear viscoelastic media, Ardéshir Guran, A. Bostrom, Herbert Überall, O. Leroy, Acoustic Interactions with Submerged Elastic Structures: Nondestructive testing, acoustic wave propagation and scattering,weblink 134, 981-02-4271-9, 1996, World Scientific, The sine function is periodic, so the sine wave or sinusoid has a wavelength in space and a period in time.BOOK,weblink 106, Aleksandr Tikhonovich Filippov, The versatile soliton, 2000, Springer, 0-8176-3635-8, BOOK, An introduction to seismology, earthquakes, and earth structure, Seth Stein, Michael E. Wysession, 31,weblink 0-86542-078-5, 2003, Wiley-Blackwell, The sinusoid is defined for all times and distances, whereas in physical situations we usually deal with waves that exist for a limited span in space and duration in time. An arbitrary wave shape can be decomposed into an infinite set of sinusoidal waves by the use of Fourier analysis. As a result, the simple case of a single sinusoidal wave can be applied to more general cases.BOOK, op. cit., Seth Stein, Michael E. Wysession, 32,weblink 0-86542-078-5, 2003, BOOK, Electromagnetic Radiation: Variational Methods, Waveguides and Accelerators, Kimball A. Milton, Julian Seymour Schwinger,weblink 16, 3-540-29304-3, Springer, 2006, Thus, an arbitrary function f(r, t) can be synthesized by a proper superposition of the functions exp[i (k·r−ωt)]...,
In particular, many media are linear, or nearly so, so the calculation of arbitrary wave behavior can be found by adding up responses to individual sinusoidal waves using the superposition principle to find the solution for a general waveform.
BOOK,weblink 433, Principles of physics, Raymond A. Serway, John W. Jewett, yes, §14.1 The Principle of Superposition, 0-534-49143-X, 2005, 4th, Cengage Learning,
When a medium is nonlinear, the response to complex waves cannot be determined from a sine-wave decomposition.

Plane waves

{{summarize|section=y|from|Plane wave|date=July 2015}}

Standing waves

File:Standing wave.gif|thumb|right|300px|Standing wave in stationary medium. The red dots represent the wave nodes ]]A standing wave, also known as a stationary wave, is a wave that remains in a constant position. This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling in opposite directions.The sum of two counter-propagating waves (of equal amplitude and frequency) creates a standing wave. Standing waves commonly arise when a boundary blocks further propagation of the wave, thus causing wave reflection, and therefore introducing a counter-propagating wave. For example, when a violin string is displaced, transverse waves propagate out to where the string is held in place at the bridge and the nut, where the waves are reflected back. At the bridge and nut, the two opposed waves are in antiphase and cancel each other, producing a node. Halfway between two nodes there is an antinode, where the two counter-propagating waves enhance each other maximally. There is no net propagation of energy over time.Image:Standing waves on a string.gif|One-dimensional standing waves; the fundamental mode and the first 5 overtones.Image:Drum vibration mode01.gif|A two-dimensional standing wave on a disk; this is the fundamental mode.Image:Drum vibration mode21.gif|A standing wave on a disk with two nodal lines crossing at the center; this is an overtone.

Physical properties

(File:Light dispersion of a mercury-vapor lamp with a flint glass prism IPNr°0125.jpg|thumb|right|upright|Light beam exhibiting reflection, refraction, transmission and dispersion when encountering a prism)Waves exhibit common behaviors under a number of standard situations, e. g.

Transmission and media

Waves normally move in a straight line (i.e. rectilinearly) through a transmission medium. Such media can be classified into one or more of the following categories:
  • A bounded medium if it is finite in extent, otherwise an unbounded medium
  • A linear medium if the amplitudes of different waves at any particular point in the medium can be added
  • A uniform medium or homogeneous medium if its physical properties are unchanged at different locations in space
  • An anisotropic medium if one or more of its physical properties differ in one or more directions
  • An isotropic medium if its physical properties are the same in all directions


Absorption of waves means, if a kind of wave strikes a matter, it will be absorbed by the matter. When a wave with that same natural frequency impinges upon an atom, then the electrons of that atom will be set into vibrational motion. If a wave of a given frequency strikes a material with electrons having the same vibrational frequencies, then those electrons will absorb the energy of the wave and transform it into vibrational motion.


When a wave strikes a reflective surface, it changes direction, such that the angle made by the incident wave and line normal to the surface equals the angle made by the reflected wave and the same normal line.


(File:Wave refraction.gif|thumb|right|200 px|Sinusoidal traveling plane wave entering a region of lower wave velocity at an angle, illustrating the decrease in wavelength and change of direction (refraction) that results.)Refraction is the phenomenon of a wave changing its speed. Mathematically, this means that the size of the phase velocity changes. Typically, refraction occurs when a wave passes from one medium into another. The amount by which a wave is refracted by a material is given by the refractive index of the material. The directions of incidence and refraction are related to the refractive indices of the two materials by Snell's law.


A wave exhibits diffraction when it encounters an obstacle that bends the wave or when it spreads after emerging from an opening. Diffraction effects are more pronounced when the size of the obstacle or opening is comparable to the wavelength of the wave.


Waves that encounter each other combine through superposition to create a new wave called an interference pattern. Important interference patterns occur for waves that are in phase.


(File:Circular.Polarization.Circularly.Polarized.Light Circular.Polarizer Creating.Left.Handed.Helix.View.svg|thumb|left)The phenomenon of polarization arises when wave motion can occur simultaneously in two orthogonal directions. Transverse waves can be polarized, for instance. When polarization is used as a descriptor without qualification, it usually refers to the special, simple case of linear polarization. A transverse wave is linearly polarized if it oscillates in only one direction or plane. In the case of linear polarization, it is often useful to add the relative orientation of that plane, perpendicular to the direction of travel, in which the oscillation occurs, such as "horizontal" for instance, if the plane of polarization is parallel to the ground. Electromagnetic waves propagating in free space, for instance, are transverse; they can be polarized by the use of a polarizing filter.Longitudinal waves, such as sound waves, do not exhibit polarization. For these waves there is only one direction of oscillation, that is, along the direction of travel.


(File:Light dispersion conceptual waves.gif|thumb|right|270 px|Schematic of light being dispersed by a prism. Click to see animation.)A wave undergoes dispersion when either the phase velocity or the group velocity depends on the wave frequency.Dispersion is most easily seen by letting white light pass through a prism, the result of which is to produce the spectrum of colours of the rainbow. Isaac Newton performed experiments with light and prisms, presenting his findings in the Opticks (1704) that white light consists of several colours and that these colours cannot be decomposed any further.BOOK, Newton, Isaac, 1704, Isaac Newton, Opticks: Or, A treatise of the Reflections, Refractions, Inflexions and Colours of Light. Also Two treatises of the Species and Magnitude of Curvilinear Figures, 118, London, Prop VII Theor V, All the Colours in the Universe which are made by Light... are either the Colours of homogeneal Lights, or compounded of these..., 1,weblink

Mechanical waves

Waves on strings

The speed of a transverse wave traveling along a vibrating string ( v ) is directly proportional to the square root of the tension of the string ( T ) over the linear mass density ( μ ):
v=sqrt{frac{T}{mu}}, ,where the linear density μ is the mass per unit length of the string.

Acoustic waves

Acoustic or sound waves travel at speed given by
v=sqrt{frac{B}{rho_0}}, ,or the square root of the adiabatic bulk modulus divided by the ambient fluid density (see speed of sound).

Water waves

(File:Shallow water wave.gif|thumb|right|400px)
  • Ripples on the surface of a pond are actually a combination of transverse and longitudinal waves; therefore, the points on the surface follow orbital paths.
  • Sound—a mechanical wave that propagates through gases, liquids, solids and plasmas;
  • Inertial waves, which occur in rotating fluids and are restored by the Coriolis effect;
  • Ocean surface waves, which are perturbations that propagate through water.

Seismic waves

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions that give out low-frequency acoustic energy.

Shock waves

(File:Transonico-en.svg|thimb|right|300 px|Formation of a shock wave by a plane.)A shock wave is a type of propagating disturbance. When a wave moves faster than the local speed of sound in a fluid, it is a shock wave. Like an ordinary wave, a shock wave carries energy and can propagate through a medium; however, it is characterized by an abrupt, nearly discontinuous change in pressure, temperature and density of the medium.{{Citation
| last = Anderson | first = John D. Jr.
| title = Fundamentals of Aerodynamics | origyear = 1984 | edition = 3rd
| publisher = McGraw-Hill Science/Engineering/Math |date=January 2001
| isbn = 0-07-237335-0 }}
{{See also|Sonic boom|Cherenkov radiation}}


  • Waves of traffic, that is, propagation of different densities of motor vehicles, and so forth, which can be modeled as kinematic waves
JOURNAL, M. J. Lighthill, James Lighthill, G. B. Whitham, Gerald B. Whitham, 1955, On kinematic waves. II. A theory of traffic flow on long crowded roads, Proceedings of the Royal Society of London. Series A, 229, 281–345, harv, ., 1955RSPSA.229..281L, 10.1098/rspa.1955.0088, And: JOURNAL, 10.1287/opre.4.1.42, P. I. Richards, 1956, Shockwaves on the highway, Operations Research, 4, 1, 42–51, harv, .,
  • Metachronal wave refers to the appearance of a traveling wave produced by coordinated sequential actions.

Electromagnetic waves

(File:Onde electromagnétique.png|thumb|right|300 px)An electromagnetic wave consists of two waves that are oscillations of the electric and magnetic fields. An electromagnetic wave travels in a direction that is at right angles to the oscillation direction of both fields. In the 19th century, James Clerk Maxwell showed that, in vacuum, the electric and magnetic fields satisfy the wave equation both with speed equal to that of the speed of light. From this emerged the idea that light is an electromagnetic wave. Electromagnetic waves can have different frequencies (and thus wavelengths), giving rise to various types of radiation such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and Gamma rays.

Quantum mechanical waves

{{See also|Wave function}}

Schrödinger equation

The Schrödinger equation describes the wave-like behavior of particles in quantum mechanics. Solutions of this equation are wave functions which can be used to describe the probability density of a particle.

Dirac equation

The Dirac equation is a relativistic wave equation detailing electromagnetic interactions. Dirac waves accounted for the fine details of the hydrogen spectrum in a completely rigorous way. The wave equation also implied the existence of a new form of matter, antimatter, previously unsuspected and unobserved and which was experimentally confirmed. In the context of quantum field theory, the Dirac equation is reinterpreted to describe quantum fields corresponding to spin-½ particles.(File:Wave packet (dispersion).gif|thumb|A propagating wave packet; in general, the envelope of the wave packet moves at a different speed than the constituent waves.BOOK, Quantum Mechanics for Applied Physics and Engineering, A. T. Fromhold, Wave packet solutions, 59 ff, (p. 61) ...the individual waves move more slowly than the packet and therefore pass back through the packet as it advances,weblink 0-486-66741-3, Courier Dover Publications, 1991, Reprint of Academic Press 1981, )

de Broglie waves

Louis de Broglie postulated that all particles with momentum have a wavelength
lambda = frac{h}{p},
where h is Planck's constant, and p is the magnitude of the momentum of the particle. This hypothesis was at the basis of quantum mechanics. Nowadays, this wavelength is called the de Broglie wavelength. For example, the electrons in a CRT display have a de Broglie wavelength of about 10−13 m.'''A wave representing such a particle traveling in the k-direction is expressed by the wave function as follows:
psi (mathbf{r}, t=0) =A e^{imathbf{k cdot r}} ,
where the wavelength is determined by the wave vector k as:
lambda = frac {2 pi}{k} ,
and the momentum by:
mathbf{p} = hbar mathbf{k} .
However, a wave like this with definite wavelength is not localized in space, and so cannot represent a particle localized in space. To localize a particle, de Broglie proposed a superposition of different wavelengths ranging around a central value in a wave packet,BOOK, Advances in Electronics and Electron Physics, 271,weblink 0-12-014653-3, 1980, Academic Press, 53, L. Marton, Claire Marton, Ming Chiang Li, Electron Interference,
a waveform often used in quantum mechanics to describe the wave function of a particle. In a wave packet, the wavelength of the particle is not precise, and the local wavelength deviates on either side of the main wavelength value.
In representing the wave function of a localized particle, the wave packet is often taken to have a Gaussian shape and is called a Gaussian wave packet.See for example BOOK,weblink Quantum Mechanics, Walter Greiner, D. Allan Bromley, 60, 3-540-67458-6, 2, 2007, Springer, and BOOK, Electronic basis of the strength of materials, John Joseph Gilman,weblink 57, 2003, 0-521-62005-8, Cambridge University Press, ,BOOK, Principles of quantum mechanics, Donald D. Fitts,weblink 17, 0-521-65841-1, Cambridge University Press, 1999, .
Gaussian wave packets also are used to analyze water waves.
BOOK,weblink 47, Chiang C. Mei, Chiang C. Mei, The applied dynamics of ocean surface waves, 9971-5-0789-7, 1989, 2nd, World Scientific, For example, a Gaussian wavefunction ψ might take the form:BOOK, Quantum Mechanics, Walter Greiner, D. Allan Bromley, 60,weblink 2nd, 2007, Springer, 3-540-67458-6,
psi(x, t=0) = A exp left( -frac{x^2}{2sigma^2} + i k_0 x right) ,
at some initial time t = 0, where the central wavelength is related to the central wave vector k0 as λ0 = 2π / k0. It is well known from the theory of Fourier analysis,BOOK, 23,weblink The picture book of quantum mechanics, Siegmund Brandt, Hans Dieter Dahmen, 0-387-95141-5, 2001, 3rd, Springer,
or from the Heisenberg uncertainty principle (in the case of quantum mechanics) that a narrow range of wavelengths is necessary to produce a localized wave packet, and the more localized the envelope, the larger the spread in required wavelengths. The Fourier transform of a Gaussian is itself a Gaussian.
BOOK, Modern mathematical methods for physicists and engineers, Cyrus D. Cantrell, 677,weblink 0-521-59827-3, Cambridge University Press, 2000,
Given the Gaussian:

f(x) = e^{-x^2 / (2sigma^2)} ,
the Fourier transform is:
tilde{ f} (k) = sigma e^{-sigma^2 k^2 / 2} .
The Gaussian in space therefore is made up of waves:
f(x) = frac{1}{sqrt{2 pi}} int_{-infty}^{infty} tilde{f} (k) e^{ikx} dk ;
that is, a number of waves of wavelengths λ such that kλ = 2 π.The parameter σ decides the spatial spread of the Gaussian along the x-axis, while the Fourier transform shows a spread in wave vector k determined by 1/σ. That is, the smaller the extent in space, the larger the extent in k, and hence in λ = 2π/k.File:GravitationalWave CrossPolarization.gif|thumb|right|Animation showing the effect of a cross-polarized gravitational wave on a ring of test particlestest particles

Gravity waves

Gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. A ripple on a pond is one example.

Gravitational waves

Gravitational waves also travel through space. The first observation of gravitational waves was announced on 11 February 2016.WEB, Gravitational waves detected for 1st time, 'opens a brand new window on the universe',weblink CBC, 11 February 2016, Gravitational waves are disturbances in the curvature of spacetime, predicted by Einstein's theory of general relativity.

See also

Waves in general

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Electromagnetic waves

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In fluids

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In quantum mechanics

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In relativity

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Other specific types of waves

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Related topics

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  • BOOK, Fleisch, D., Kinnaman, L., 2015, A student's guide to waves, Cambridge, UK, Cambridge University Press, 978-1107643260, harv, .,
  • BOOK, Campbell, Murray, The musician's guide to acoustics, 2001, Oxford University Press, Oxford, 978-0198165057, Repr., Greated, Clive,
  • BOOK, A.P., French, Vibrations and Waves (M.I.T. Introductory physics series), Nelson Thornes, 1971, 0-393-09936-9, 163810889,
  • BOOK, Hall, D. E., 1980, Musical Acoustics: An Introduction, Belmont, California, Wadsworth Publishing Company, 0-534-00758-9, harv, ., .
  • BOOK, Hunt, Frederick Vinton, Origins in acoustics., 1978, Published for the Acoustical Society of America through the American Institute of Physics, Woodbury, NY, 978-0300022209,
  • BOOK, Ostrovsky, L. A., Potapov, A. S., 1999, Modulated Waves, Theory and Applications, Baltimore, The Johns Hopkins University Press, 0-8018-5870-4, harv, ., .
  • BOOK, Academic Press, 9780123846532, Griffiths, G., W. E., Schiesser, Traveling Wave Analysis of Partial Differential Equations: Numerical and Analytical Methods with Matlab and Maple, 2010,

External links

{{Sister project links| wikt=wave | commons=Wave | b=no | n=no | q=Wave | s=no | v=no | voy=no | species=no | d=no}} {{Velocities of Waves}}{{Patterns in nature}}{{Authority control}}

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M.R.M. Parrott