A wave is a disturbance that propagates through space and time, usually with transference of energy. A mechanical wave is a wave that propagates or travels through a medium due to the restoring forces it produces upon deformation. For example, when a sound wave is traveling through the air, air molecules slam into their neighbors, which pushes their neighbors into their neighbors (and so on); but when air molecules collide with their neighbors, they also bounce away from them back in the direction they came from. These collisions provide a restoring force that keeps the molecules from actually traveling with the wave.
Waves travel and transfer energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass transport; they consist instead of oscillations or vibrations around almost fixed locations. In the picture of water waves, if we imagine a cork on the water, it would bob up and down staying in about the same place, although the wave itself is moving outward. When we say that a wave carries energy but not mass, we are referring to this fact that even as the wave travels outward from the center (carrying energy of motion), the medium itself does not flow with it.
In many areas of science, the idea of a wave is used metaphorically. If an ocean wave is seen as a prototype wave, it is the basis for the metaphor—the surface of water undulating up and down. However, upon investigating a sound wave, its air does not undulate up and down (as the ocean surface did). Instead, an abstraction is made; if we could look at the air molecules, they would be bunching together (in compressions) and then spreading apart (in rarefactions). Thus, the medium itself is not undulating up and down, but its density is (and its pressure is). When we speak of waves in physics, therefore, we are often speaking metaphorically, in an abstraction, of a periodic fluctuation of a specific characteristic. The characteristics that oscillate could be density, pressure, electrical or magnetic polarities or other (sometimes exotic) characteristics.
There are also waves capable of traveling through a vacuum, including electromagnetic radiation. Ultraviolet radiation, infrared radiation, gamma rays, Xrays, and radio waves are examples of these types of waves. They consist of period oscillations in electrical and magnetic properties that grow, reach a peak, diminish, go to zero, and then continue these changes in a periodic fashion.
As well, it is believed that gravitational waves travel through space; gravitational waves have never been directly detected but are believed to exist. (See gravitational radiation.)
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Agreeing on a single, allencompassing definition for the term wave is nontrivial. A vibration can be defined as a backandforth motion around a reference value. However, a vibration is not necessarily a wave. Defining the necessary and sufficient characteristics that qualify a phenomenon to be called a wave is, at least, flexible.
The term is often understood intuitively as the transport of disturbances in space, not associated with motion of the 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 (Hall 1980, p. 8). However, this notion is problematic for a standing wave (for example, a wave on a string), where energy is moving in both directions equally, or for electromagnetic / light waves in a vacuum, where the concept of medium does not apply. There are water waves in the ocean; light waves from the sun; microwaves inside the microwave oven; radio waves transmitted to the radio; and sound waves from the radio, telephone, and voices.
It may be seen that the description of waves is accompanied by a heavy reliance on physical origin when describing any 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 wavelike transfer / transformation of vibratory energy. 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, etc., and so on).
Other properties, however, although they are usually described in an originspecific manner, 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 from their physical origin.^{[1]} For example, based on the mechanical origin of acoustic waves there can be a moving disturbance in space–time 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.
Similarly, wave processes revealed from the study of waves other than sound waves can be significant to the understanding of sound phenomena. A relevant example is Thomas Young's principle of interference (Young, 1802, in Hunt 1992, p. 132). This principle was first introduced in Young's study of light and, within some specific contexts (for example, scattering of sound by sound), is still a researched area in the study of sound.
Periodic waves are characterized by crests (highs) and troughs (lows), and may usually be categorized as either longitudinal or transverse. Transverse waves are those with vibrations perpendicular to the direction of the propagation of the wave; examples include waves on a string, and electromagnetic waves. Longitudinal waves are those with vibrations parallel to the direction of the propagation of the wave; examples include most sound waves.
When an object bobs up and down on a ripple in a pond, it experiences an orbital trajectory because ripples are not simple transverse sinusoidal waves.
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.
All waves have common behavior under a number of standard situations. All waves can experience the following:
A wave is polarized if it oscillates in one direction or plane. A wave can be polarized by the use of a polarizing filter. The polarization of a transverse wave describes the direction of oscillation in the plane perpendicular to the direction of travel.
Longitudinal waves such as sound waves do not exhibit polarization. For these waves the direction of oscillation is along the direction of travel.this is very important
Examples of waves include:
Mathematically, the most basic wave is the sine wave (or harmonic wave or sinusoid), with an amplitude u described by the equation:
where A is the semiamplitude of the wave, half the peaktopeak amplitude, often called simply the amplitude – the 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 (spatial frequency), ω is the temporal frequency, and φ is a phase offset.
The units of the semiamplitude depend on the type of wave — waves on a string have an amplitude expressed as a distance (meters), sound waves as pressure (pascals) and electromagnetic waves as the amplitude of the electric field (volts/meter).
The wavelength (denoted as λ) is the distance between two sequential crests (or troughs), and 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
The period T is the time for one complete cycle of an oscillation of a wave. The frequency f (also frequently denoted as ν ) is the number of periods per unit time (per second) and is measured in hertz. These are related by:
In other words, the frequency and period of a wave are reciprocals.
The angular frequency ω represents the frequency in radians per second. It is related to the frequency by
The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by:^{[4]}
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, shown in the figure, 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.^{[3]}
Although arbitrary wave shapes will propagate unchanged in lossless linear timeinvariant 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.^{[5]} 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.^{[6]} The sine function is periodic, so the sine wave or sinusoid has a wavelength in space and a period in time.^{[7]}^{[8]}
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. Fortunately, 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.^{[9]}^{[10]} 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.^{[11]} When a medium is nonlinear, the response to complex waves cannot be determined from a sinewave decomposition.
The wave equation is a partial differential equation that describes the evolution of a wave over time in a medium where the wave propagates at the same speed independent of wavelength (no dispersion), and independent of amplitude (linear media, not nonlinear).^{[12]} General solutions are based upon Duhamel's principle.^{[13]}
In particular, consider the wave equation in one dimension, for example, as applied to a string. Suppose a onedimensional wave is traveling along the x axis with velocity v and amplitude u (which generally depends on both x and t), the wave equation is
The velocity v will depend on the medium through which the wave is moving.
The general solution for the wave equation in one dimension was given by d'Alembert; it is known as d'Alembert's formula:^{[14]}
This formula represents two shapes traveling through the medium in opposite directions; F in the positive x direction, and G in the negative x direction, of arbitrary functional shapes F and G.
The form of the forward propagating wave 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 xdirection at velocity v (and G will propagate at the same speed in the negative xdirection).^{[16]}
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 (sometimes called 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.^{[15]}
To summarize:
"A function F (x) is periodic if F(x+ξ) = F(x), for all x. The constant ξ is called a period of the function. The smallest such period is called the fundamental period or simply the period of F. If x represents a space coordinate, then the period may instead be called the wavelength and is often written λ; if it represents the time coordinate, the period might instead be denoted by T." Flowers, p. 473^{[17]}
The Schrödinger equation describes the wavelike 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. Quantum mechanics also describes particle properties that other waves, such as light and sound, have on the atomic scale and below.
Louis de Broglie postulated that all particles with momentum have a wavelength
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 kdirection is expressed by the wave function:
where the wavelength is determined by the wave vector k as:
and the momentum by:
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,^{[19]} 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.^{[20]} Gaussian wave packets also are used to analyze water waves.^{[21]}
For example, a Gaussian wavefunction ψ might take the form:^{[22]}
at some initial time t = 0, where the central wavelength is related to the central wave vector k_{0} as λ_{0} = 2π / k_{0}. It is well known from the theory of Fourier analysis,^{[23]} 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.^{[24]} Given the Gaussian:
the Fourier transform is:
The Gaussian in space therefore is made up of waves:
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 xaxis, 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.
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:^{[25]}^{[26]}^{[27]}
where is the amplitude envelope of the wave, k is the wave number and φ is the phase. If the group velocity (see below) is wavelength independent, this equation can be simplified as:^{[28]}
where v_{g} is the group velocity, showing that the envelope moves with velocity v_{g} 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.^{[28]}^{[29]}
There are two velocities that are associated with waves, the phase velocity and the group velocity. To understand them, one must consider several types of waveform. For simplification, examination is restricted to one dimension.
The most basic wave (a form of plane wave) may be expressed in the form:
which can be related to the usual sine and cosine forms using Euler's formula. Rewriting the argument, (kx −ωt) = (2π/λ)(x − vt), makes clear that this expression describes a vibration of wavelength λ = 2π/k traveling in the xdirection with a constant phase velocity v_{p}:^{[30]}
The other type of wave to be considered is one with localized structure described by an envelope, which may be expressed mathematically as, for example:
where now A(k_{1}) (the integral is the inverse fourier transform of A(k1)) is a function exhibiting a sharp peak in a region of wave vectors Δk surrounding the point k_{1} = k. In exponential form:
with A_{o} the magnitude of A. For example, a common choice for A_{o} is a Gaussian wave packet:^{[31]}
where σ determines the spread of k_{1}values about k, and N is the amplitude of the wave.
The exponential function inside the integral for ψ oscillates rapidly with its argument, say φ(k_{1}), and where it varies rapidly, the exponentials cancel each other out, interfere destructively, contributing little to ψ.^{[30]} However, an exception occurs at the location where the argument φ of the exponential varies slowly. (This observation is the basis for the method of stationary phase for evaluation of such integrals.^{[32]}) The condition for φ to vary slowly is that its rate of change with k_{1} be small; this rate of variation is:^{[30]}
where the evaluation is made at k_{1} = k because A(k_{1}) is centered there. This result shows that the position x where the phase changes slowly, the position where ψ is appreciable, moves with time at a speed called the group velocity:
The group velocity therefore depends upon the dispersion relation connecting ω and k. For example, in quantum mechanics the energy E = ħω = (ħk)^{2}/(2m). Consequently,
showing that the velocity of a localized particle in quantum mechanics is its group velocity.^{[30]} Because the group velocity varies with k, the shape of the wave packet broadens with time, and the particle becomes less localized.^{[33]} In other words, the velocity of the constituent waves of the wave packet travel at a rate that varies with their wavelength, so some move faster than others, and they cannot maintain the same interference pattern as the wave propagates.
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 counterpropagating 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 counterpropagating wave. For example when a violin string is displaced, longitudinal waves propagate out to where the string is held in place at the bridge and the "nut", whereupon 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 counterpropagating waves enhance each other maximally. There is on average no net propagation of energy.
Also see: Acoustic resonance, Helmholtz resonator, and organ pipe
The speed of a 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):
where the linear density μ is the mass per unit length of the string.
The medium that carries a wave is called a transmission medium. It can be classified into one or more of the following categories:
In a nonuniform medium, in which the wavenumber k can depend on the location as well as the frequency, the phase term kx is typically replaced by the integral of k(x)dx, according to the WKB method. Such nonuniform traveling waves are common in many physical problems, including the mechanics of the cochlea and waves on hanging ropes.
Velocities of Waves 

Phase velocity  Group velocity  Front velocity  Signal velocity 
The
Wave: An Egyptian Aftermath by 
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This work is in the public domain in
the United States because it was published before
January 1, 1923.
The author died in 1951, so this work is also in the public domain in countries and areas where the copyright term is the author's life plus 50 years or less. This work may also be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works. 
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