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Wave#Travelling waves
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{{About|waves in the scientific sense|waves on seas and lakes|wind wave|other uses|wave (disambiguation)}}(File:2006-01-14 Surface waves.jpg|thumb|Surface waves in water showing water ripples)(File:Rectified waves.png|thumb|Different types of wave with varying rectifications)In physics, mathematics, and related fields, a wave is a disturbance of a field in which a physical attribute oscillates repeatedly at each point or propagates from each point to neighboring points, or seems to move through space.The waves most commonly studied in physics are mechanical and electromagnetic. A mechanical wave is a local deformation (strain) in some physical medium that propagates from particle to particle by creating local stresses that cause strain in neighboring particles too. For example, sound waves in air are variations of the local pressure that propagate by collisions between gas molecules. Other examples of mechanical waves are seismic waves, gravity waves, vortices, and shock waves. An electromagnetic wave consists of a combination of variable electric and magnetic fields, that propagates through space according to Maxwell's equations. Electromagnetic waves can travel through suitable dielectric media or through vacuum; examples include radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Other types of waves include gravitational waves, which are disturbances in a gravitational field that propagate according to general relativity; heat diffusion waves; plasma waves, that combine mechanical deformations and electromagnetic fields; reaction-diffusion waves, such as in the Belousovâ€“Zhabotinsky reaction; and many more.Mechanical and electromagnetic waves may often seem to travel through space; but, while they can carry energy,{{Harv|Hall|1982| p=8}} momentum, and information through matter or empty space, they may do that without transferring any mass. In mathematics and electronics waves are studied as signals.Pragnan Chakravorty, "What Is a Signal? [Lecture Notes],"Â IEEE Signal Processing Magazine, vol. 35, no. 5, pp. 175-177, Sept. 2018.Â {{doi.org|10.1109/MSP.2018.2832195}} On the other hand, some waves do not appear to move at all, like standing waves (which are fundamental to music) and hydraulic jumps. Some, like the probability waves of quantum mechanics, may be completely static.A physical wave 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 seems to travel in a definite direction, and has constant value over any plane perpendicular to that direction. Mathematically, the simplest waves are the sinusoidal ones. Complicated waves can often be described as the sum of many sinusoidal plane waves. A plane wave can be transverse, if its effect at each point is described by a vector that is perpendicular to the direction of propagation or energy transfer; or longitudinal, if the describing vectors are parallel to the direction of energy propagation. While mechanical waves can be both transverse and longitudinal, electromagnetic waves are transverse in free space.

Mathematical Description

Single waves

A wave can be described just like a field, namely as a function F(x,t) where x is a position and t is a time. The value of x is a point of space, specifically in the region where the wave is defined. In mathematical terms, it is usually a vector in the Cartesian three-dimensional space mathbb{R}^3. However, in many casesone can ignore one or two dimensions, and let x be a point of the Cartesian plane mathbb{R}^2. This is the case, for example, when studying vibrations of a drum skin). One may even restrict x to a point of the Cartesian line mathbb{R}^2 â€” that is, the set of real numbers. This is the case, for example, when studying vibrations in a violin string or recorder. The time t, on the other hand, is always assumed to be a scalar; that is, a real number.The value of F(x,t) can be any physical quantity of interest assigned to the point x that may vary with time. For example, if F represents the vibrations inside an elastic solid, the value of F(x,t) is usually a vector that gives the current displacement from x of the material particles that would be at the point x in the absence of vibration. For an electromagnetic wave, the value of F can be the electric field vector E, or the magnetic field vector H, or any related quantity, such as the Poynting vector Etimes H. In fluid dynamics, the value of F(x,t) could be the velocity vector of the fluid at the point x, or any scalar property like pressure, temperature, or density. In a chemical reaction, F(x,t) could be the concentration of some substance in the neighborhood of point x of the reaction medium.For any dimension d (1, 2, or 3), the wave's domain is then a subset D of mathbb{R}^d, such that the function value F(x,t) is defined for any point x in D. For example, when describing the motion of a drum skin, one can consider D to be a disk (circle) on the plane mathbb{R}^2 with center at the origin (0,0), and let F(x,t) be the vertical displacement of the skin at the point x of of D and at time t.

Wave families

Sometimes one is interested in a single specific wave, like how the Earth vibrated after the 1929 Murchison earthquake. More often, however, one needs to understand large set of possible waves; like all the ways that a drum skin can vibrate after being struck once with a drum stick, or all the possible radar echos one could get from an airplane that may be approaching an airport. In some of those situations, one may describe such a family of waves by a function F(A,B,ldots;x,t) that depends on certain parameters A,B,ldots, besides x and t. Then one can obtain different waves â€” that is, different functions of x and t â€” by choosing different values for those parameters.(File:Half-open pipe wave.gif|thumb|right|Sound pressure wave in an half-open pipe playing the 7th harmonic of the fundamental (n = 4).)For example, the sound pressure inside a recorder that is playing a "pure" note is typically a standing wave, that can be written as
F(A,L,n,c;x,t) = A,(cos 2pi xfrac{2 n - 1}{4 L}) (cos 2pi c tfrac{2n - 1}{4 L})
The parameter A defines the amplitude of the wave (that is, the maximum sound pressure in the bore, which is related to the loudness of the note); c is the speed of sound; L is the length of the bore; and n is a positive integer (1,2,3,...) that specifies the number of nodes in the standing wave. (The position x should be masured from the mouthpiece, and the time t from any moment at which the pressure at the mouthpiece is maximum. The quantity lambda = 4L/(2 n - 1) is the wavelength of the emitted note, and f = c/lambda is its frequency.) Many general properties of these waves can be inferred from this general equation, without choosing specific values for the parameters.As another example, it may be that the vibrations of a drum skin after a single strike depend only on the distance r from the center of the skin to the strike point, and on the strength s of the strike. Then the vibration for all possible strikes can be described by a function F(r,s;x,t).Sometimes the family of waves of interest has infinitely many parameters. For example, one may want to describe what happens to the temperature in a metal bar when it is initially heated at various temperatures at different points along its length, and then allowed to cool by itself in vacuum. In that case, instead of a scalar or vector, the parameter would have to be a function h such that h(x) is the initial temperature at each point x of the bar. Then the temperatures at later times can be expressed by a function F that depends on the function h (that is, a functional operator), so that the temperature at a later time is F(h;x,t)

Differential wave equations

Another way to describe and study a family of waves is to give a mathematical equation that, instead of explictly giving the value of F(x,t), only constrains how those values can change with time. Then the family of waves in question consists of all functions F that satisfy those constraints â€” that is, all solutions of the equation.This approach is extremely important in physics, because the constraints usually are a consequence of the physical processes that cause the wave to evolve. For example, if F(x,t) is the temperature inside a block of some homogeneous and isotropic solid material, its evolution is constrained by the partial differential equation
frac{partial F}{partial t}(x,t) = alpha left(frac{partial^2 F}{partial x_1^2}(x,t) + frac{partial^2 F}{partial x_2^2}(x,t) + frac{partial^2 F}{partial x_3^2}(x,t) right) + beta Q(x,t)
where Q(p,f) is the heat that is being generated per unit of volume and time in the neighborhood of x at time t (for example, by chemical reactions happening there); x_1,x_2,x_3 are the Cartesian coordinates of the point x; partial F/partial t is the (first) derivative of F with respect to t; and partial^2 F/partial x_i^2 is the second derivative of F relative to x_i. (The simbol "partial" is meant to signify that, in the derivative with respect to some variable, all other variables must be considered fixed.)This equation can be derived from the laws of physics that govern the diffusion of heat in solid media. For that reason, it is called the heat equation in mathematics, even though it applies to many other physical quantities besides temperatures.For another example, we can describe all possible sounds echoing within a container of gas by a function F(x,t) that gives the pressure at a point x and time t within that container. If the gas was initially at uniform temperature and composition, the evolution of F is constrained by the formula
frac{partial^2 F}{partial t^2}(x,t) = alpha left(frac{partial^2 F}{partial x_1^2}(x,t) + frac{partial^2 F}{partial x_2^2}(x,t) + frac{partial^2 F}{partial x_3^2}(x,t) right) + beta P(x,t)
Here P(x,t) is some extra compression force that is being applied to the gas near x by some external process, such as a loudspeaker or piston] right next to p.This same differential equation describes the behavior of mechanical vibrations and electromagnetic fields in a homogeneous isotropic non-conducting solid. Note that this equation differs from that of heat flow only in that the left-hand side is partial^2 F/partial t^2, the second derivative of with respect to time, rather than the first derivative partial F/partial t. Yet this small change makes a huge difference on the set of solutions F. This diferential equation is called "the" wave equation in mathematics, even though it describes only one very special kind of waves.

Wave in elastic medium

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 Î», 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. For example, 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, 978-0-08-043930-3, 2003, Elsevier, BOOK, Modulated waves: theory and application,weblink Lev A. Ostrovsky, Alexander I. Potapov, yes, Johns Hopkins University Press, 978-0-8018-7325-6, 2001,
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 13â€“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, 37ff, 978-0-8218-2749-9, 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 âˆ’ 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, 978-0-521-43601-4, Cambridge University Press, 1998, In the case of a periodic function F with period Î», that is, F(x + Î» âˆ’ vt) = F(x âˆ’ 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 Î» (the wavelength of the wave). In a similar fashion, this periodicity of F implies a periodicity in time as well: F(x âˆ’ v(t + T)) = F(x âˆ’ vt) provided vT = Î», so an observation of the wave at a fixed location x finds the wave undulating periodically in time with period T = Î»/v.BOOK, Introduction to Macromolecular Crystallography, Alexander McPherson,weblink 77, Waves and their properties, 978-0-470-18590-2, 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, 978-3-86537-419-6, 2005, Cuvillier Verlag, BOOK, Oscillations and waves, Fritz Kurt KneubÃ¼hl,weblink 365, 1997, 978-3-540-62001-3, Springer, BOOK,weblink 33, Mark Lundstrom, 978-0-521-63134-1, 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, Â§13.7.3 Pulse envelope in nondispersive media, 978-0-471-75687-3, 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. HernÃ¡ndez-Figueroa, Michel Zamboni-Rached, Erasmo Recami,weblink 329, 978-0-470-10885-7, 2008, Wiley-Interscience,

Phase velocity and group velocity

missing image!
- Wave group.gif -
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|Î»}} (lambda) and period {{mvar|T}} as
v_mathrm{p} = frac{lambda}{T}.
missing image!
- Wave opposite-group-phase-velocity.gif -
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 (that is, phase velocity) of the overall shape of the waves' amplitudes â€“ 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) ,
where
• 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 (for example, a wave on a string) have an amplitude expressed as a distance (for example, meters), longitudinal mechanical waves (for example, sound waves) use units of pressure (for example, pascals), and electromagnetic waves (a form of transverse vacuum wave) express the amplitude in terms of its electric field (for example, 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 339ff, 978-0-387-98756-9, 2002, BirkhÃ¤user,
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, 978-0-7637-5993-3, 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 Kramersâ€“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, 978-0-521-82634-1, 2005, Cambridge University Press, ; Eq. 6.36 and associated discussion in BOOK, Introduction to solid-state theory, Otfried Madelung,weblink 261 ff, 978-3-540-60443-3, 1996, 3rd, Springer, ; and Eq. 3.5 in BOOK, F Mainardi, Transient waves in linear viscoelastic media, ArdÃ©shir Guran, A. Bostrom, Herbert Ãœberall, O. Leroy, Acoustic Interactions with Submerged Elastic Structures: Nondestructive testing, acoustic wave propagation and scattering,weblink 134, 978-981-02-4271-8, 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, 978-0-8176-3635-7, BOOK, An introduction to seismology, earthquakes, and earth structure, Seth Stein, Michael E. Wysession, 31,weblink 978-0-86542-078-6, 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 978-0-86542-078-6, 2003, BOOK, Electromagnetic Radiation: Variational Methods, Waveguides and Accelerators, Kimball A. Milton, Julian Seymour Schwinger,weblink 16, 978-3-540-29304-0, Springer, 2006, Thus, an arbitrary function f(r, t) can be synthesized by a proper superposition of the functions exp[i (kÂ·râˆ’Ï‰t)]...,
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, Â§14.1 The Principle of Superposition, 978-0-534-49143-7, 2005, 4th, Cengage Learning,
When a medium is nonlinear, then 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 IPNrÂ°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, for example:

Transmission and media

Waves normally move in a straight line (that is, 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

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.

Reflection

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.

Refraction

(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.

Diffraction

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.

Interference

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.

Polarization

(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.

Dispersion

(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 ( Î¼ ):
v=sqrt{frac{T}{mu}}, ,where the linear density Î¼ 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 â€“ 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|thumb|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 = 978-0-07-237335-6 }}

Other

• 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, 1178, 281â€“345, harv, 1955RSPSA.229..281L, 10.1098/rspa.1955.0088, 10.1.1.205.4573, And: JOURNAL, 10.1287/opre.4.1.42, P.I. Richards, 1956, Shockwaves on the highway, Operations Research, 4, 1, 42â€“51, harv,
• Metachronal wave refers to the appearance of a traveling wave produced by coordinated sequential actions.

Electromagnetic waves

(File:Onde electromagnÃ©tique.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

SchrÃ¶dinger equation

The SchrÃ¶dinger 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-Â½ 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 978-0-486-66741-6, 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 10âˆ’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 978-0-12-014653-6, 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, 978-3-540-67458-0, 2, 2007, Springer, and BOOK, Electronic basis of the strength of materials, John Joseph Gilman,weblink 57, 2003, 978-0-521-62005-5, Cambridge University Press, ,BOOK, Principles of quantum mechanics, Donald D. Fitts,weblink 17, 978-0-521-65841-6, 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, 978-9971-5-0789-3, 1989, 2nd, World Scientific, For example, a Gaussian wavefunction Ïˆ might take the form:BOOK, Quantum Mechanics, Walter Greiner, D. Allan Bromley, 60,weblink 2nd, 2007, Springer, 978-3-540-67458-0,
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 Î»0 = 2Ï€ / 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, 978-0-387-95141-6, 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 978-0-521-59827-9, 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 Î» such that kÎ» = 2 Ï€.The parameter Ïƒ 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/Ïƒ. That is, the smaller the extent in space, the larger the extent in k, and hence in Î» = 2Ï€/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.

Waves in general

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Parameters

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Waveforms

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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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Sources

• BOOK, Fleisch, D., Kinnaman, L., 2015, A student's guide to waves, Cambridge, Cambridge University Press, 978-1107643260, harv, 2015sgw..book.....F,
• 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, 978-0-393-09936-2, 163810889,
• BOOK, Hall, D.E., 1980, Musical Acoustics: An Introduction, Belmont, CA, Wadsworth Publishing Company, 978-0-534-00758-4, 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, 978-0-8018-5870-3, 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,

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