Whats the differnce between phase and group velocity. Phase and group angles and velocities in anisotropic wave propagation the phase angles and velocities are different from the group angles and velocities. The wave packet envelope is shown in magenta color in dashed lines. Interplay of nonlocal response, damping, and low group velocity in.
Group and phase velocity up to now we have just talked about the speed or velocity v of a wave. General theorems on the equivalence of group velocity and. Without quantum mechanics a particle is not considered a wave so there is no group velocity or phase velocity, only the velocity of the particle. A fundamental relation between phase and group velocity, and. A fundamental relation between phase and group velocity. The wavenumber k can be considered as the change in spectral phase per unit length the group velocity is the velocity with which the envelope of a pulse propagates in a medium, assuming a long pulse with narrow bandwidth so that higherorder chromatic dispersion is not relevant and the absence of nonlinear effects i. The most familiar type of dispersion is frequency dispersion, i. This is defined by chosen here to equal 1, where is the angular velocity and is the wave number. Johnson1 1department of mathematics, massachusetts institute of technology, cambridge, massachusetts 029, usa 2center for materials science. You must there are over 200,000 words in our free online dictionary, but you are looking for one thats only in the merriam. Lets continue using the superposed wave equation from above. You can nd more information on the distinction between the phase velocity and the group velocity of a wave is a. The group velocity is simply the derivative of frequency. For quantum electron waves, the group velocity is twice the phase velocity, for water waves, its half the phase velocity.
Rp photonics encyclopedia group velocity, light, envelope. Suppose there is a series of ocean waves approaching the shore at an oblique angle that is, they are not coming in perpendicular to the shore line. It introduces three different definitions of velocities. Vary the group velocity away from unity, and carefully follow one of the packets as it moves across the screen, then carefully follow one of the individual crests.
Easy to build popups with personalization for your eshop. Does a single photon particle move in the direction of the group velocity or the wave vector. Phase velocity and group velocity in engineering physics 2 pdf free download. A propagating wave packet group velocity dispersion. Group and phase velocities in the r theory of matter waves. Fundamental relation between phase and group velocity, and. We will find that this speed is not the same as the phase velocity. Phase velocity and group velocity as a function of the total.
This notes is meant to be a supplement and not a substitute for. A onedimensional harmonic wave figure 1 is described by the equation. Phase and group velocity are two important and related concepts in wave mechanics. Phase velocity definition of phase velocity by merriamwebster. The phase velocity is the speed at which one member of the phase group travels.
Following fowler 2003, the corresponding approximation for the group velocity is the following. The ratio between the speed of light c and the phase velocity v p is known as the refractive index, n cv p ck. An important interpretation of group velocity is that it represents the velocity at which the energy or information is conveyed along a. What is the difference between phase velocity and group. In this deepwater case, the phase velocity is twice the group velocity. For quantum electron waves, the group velocity is twice the phase velocity, for water waves, its half the phase. The group velocity is the speed of the overall shape of a modulated wave called the envelope. Although we arent smart enough to bore you with the details of how this is derived order a copy of pozar from our book page, group velocity is slowed by the inverse of the factor that phase velocity is increased. The qvm technique compensates for velocity mismatching between an optical group velocity and a microwave phase velocity in the eo crystal by periodically inverting the sign of the eo coefficient of the crystal. Fundamental relation between phase and group velocity, and application to the failure of perfectly matched layers in backwardwave structures poru loh,1, ardavan f.
If the derivative decreases rapidly enough with frequency, the group velocity. The group velocity is the speed of the constructed wave, created the group of waves. Lecture 28 phys 3750 d m riffe 1 320 a propagating wave packet group velocity dispersion overview and motivation. Phase and group angles and velocities stanford university. Johnson1 1department of mathematics, massachusetts institute of technology, cambridge, ma 029. For hyperbolic systems, such as the wave equation, it must be true that the group velocity is bounded from above i. Phase velocity vs group velocity group velocity is applicable when there is information or modulation added to a carrier frequency. Group and phase velocity wolfram demonstrations project. Rarely, some people use the terms group index of refraction or group refractive index as a synonym of group index.
Interplay of nonlocal response, damping, and low group velocity in surface plasmon polaritons. Exact vti velocity formulas crewes research report volume 25 20 1 exact formulas for qp, qsv, and sh group velocities in vti media joe wong abstract for analyzing traveltimes of qp and qs arrivals propagating through media with transversely isotropic or vti symmetry, geophysicists often use the thomsen linearized. The phase velocity of a wave is the speed at which a given phase of a wave travels through space, equal to. Wave propagation and group velocity contains papers on group velocity which were published during the first world war and are missing in many libraries. A slowness vector, which is in the direction of the wavefront normal, has been selected by drawing an arrow from the origin to the dispersion curve. Phase velocity and group velocity in engineering physics 2. Refraction is the phenomenon where light bends when it enters a medium, as described by snells law. As observed experimentally the group velocity may exceed c, may be zero, or may be opposite in direction to the phase velocity. Here is a simple way to think about the difference. Phase velocity estimation of a microstrip line in a. This velocity is called the group velocity, since its the velocity of the envelope of a group in this case, 2 of waves traveling together.
Phase velocity definition is the velocity of a wave motion as determined by the product of the wavelength and frequency called also wave velocity. Johnson1 1department of mathematics, massachusetts institute of technology, cambridge, massachusetts 029, usa 2center for materials science and engineering. The velocity of the envelope function given by equation 14 is v g. Review of basic formulas for waves in spacetime x,t or perspacetime. Exact formulas for qp, qsv, and sh group velocities in vti. Group delay is the length of a circuit or transmission line, divided by its group velocity. In the ideal qvmeom, the modulation index is almost proportional to the interaction length and as a result, large modulation index at. Exact formulas for qp, qsv, and sh group velocities in vti media. I dont know the real linguistic history, but i bet that is why somebody came up with the term index of refraction or refractive index to refer to c phase velocity. The ratio between the speed of light c and the phase velocity v p is known as the refractive index, n c v p ck.
Net that is a port of the popular java project velocity. In r theory however the group velocity is defined by r and may exceed the velocity c. The group velocity of a wave is the velocity with which the overall envelope shape of the waves amplitudesknown as the modulation or envelope of the wavepropagates through space for example, if a stone is thrown into the middle of a very still pond, a circular pattern of waves with a quiescent center appears in the water, also known as a capillary wave. Kishk university of mississippi usa abstractwe study theoretically the propagation of electromagnetic waves in an in. Phase velocity and group velocity this note is part of the supporting online material for the textbook \solid state physics. As an example, if you send a pulsed carrier thru a network, the group velocity can be derived from the time delay or difference between the input rise time to the output rise time and the distance traveled. Click on the file icon or file name to start downloading. Phase velocity and group velocity for beginners in the first section of this paper i derive the formulas for the phase velocity and group velocity as a function of the total relativistic energy and the momentum of a particle. From this phase velocity, relative dielectric constant of is derived using kobayashis formula. The fact that these two velocities are different is a consequence of the fact that in a dispersive. We shall identify e with the energy density per unit distance and give a proof that in all cases considered the group velocity is given by vg ge, 2. May 21, 2016 an important interpretation of group velocity is that it represents the velocity at which the energy or information is conveyed along a wave see wikipedia for further details. What are phase velocity, group velocity, and signal velocity.
The corresponding direction of group velocity may now be determined graphically by noting that group velocity is defined by the gradient operator in equation. On measuring surface wave phase velocity from station. They arise in quantum mechanics in the time development of the state function for the continuous case, i. Phase velocity of a wave the rate at which the phase of the wave propagates in space group velocity of a wave the velocity with which the overall shape of the waves amplitudes wave packet propagates through space. Group velocity group delay is the length of a circuit or transmission line, divided by its group velocity. The threshold condition for the appearance of this effect is that the velocity of the particle coincides with the group velocity. Socital uses proprietary algorithms to determine brand affiliations, personas, and other customer characteristics, in order to match products with individual buyers. However, in the case of a superposed wave, we need to carefully define its propagation velocity. Exact expressions for the x and zcomponents r u e, u i of the group velocity in vti media can be derived from the exact expressions for phase velocity using the method of characteristics described by cerveny 2001.
Dispersive waves are waves in which the phase speed varies with wavenumber. Clearly the velocity of a monochrome light equals its equiphase surface propagation velocity. This is the velocity at which the phase of any one frequency. Particle waves and group velocity university of kansas. The group velocity of a wave is the velocity with which the overall envelope shape of the waves amplitudesknown as the modulation or envelope of the wavepropagates through space. Wavesgroup velocity wikibooks, open books for an open world. Im trying to use a classpathresourceloader for loading my. The phase velocity of a wave is the speed at which a. The wavenumber k can be considered as the change in spectral phase per unit length the group velocity is the velocity with which the envelope of a pulse propagates in a medium, assuming a long pulse with narrow bandwidth so that higherorder chromatic dispersion is not relevant and the absence of. The group velocity often depends on frequency we have seen that the phase velocity depends on, because n does. The half period of the domain inversion at the modulation frequency of near 16 ghz is derived as mm with the phase velocity of ms and the group velocity of 1.
Phase velocity and group velocity as a function of the angular frequency and the wave number 4. Group and phase velocity inversions for the general. Oskooi,2 mihai ibanescu,3 maksim skorobogatiy,4 and steven g. The amount of bending depends on the phase velocity. Nov 01, 2005 where v v, v h, v n, are respectively the vertical velocity, the horizontal velocity and the nmo velocity. The red dot overtakes two green dots when moving from the left to the ri. Next, the term group index is a combination of the word group suggesting the relation with group velocity and the word index suggesting its analogous mathematical form to refractive index.
Frequency dispersion in groups of gravity waves on the surface of deep water. In the top left is a group that moves at the same velocity as the individual wave that makes it. For the scalar wave equation,the group velocity vector and the phase velocity vector turn out to be the same, as can be verified by differentiation and substitution. The resulting wave packet travels with the socalled group velocity which in the above case is faster than the phase velocity. The group velocity of a wave is the velocity with which the overall envelope shape of the waves amplitudesknown as the modulation or envelope of the. The red dot moves with the phase velocity, and the green dots propagate with the group velocity. It is easy to show that dispersive waves have unequal phase and group velocities, while these velocities are equal for nondispersive waves. When the group velocity depends on frequency, this is known as group velocity dispersion, or gvd. The phase velocity and group velocity are speed in terms of an expression which are directly equal to each other. Under well defined conditions, superluminal signalling becomes possible using the group velocity.
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