# Questions tagged [phase-velocity]

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### Wave packet - phase vs group velocity

So, I've been revising QM recently and the concept of group vs phase velocity has me confused. Let's say we have a wave packet. As quoted in Griffiths, "...A wave packet is a superposition of ...
2answers
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### Deriving group velocity

At the introduction to quantum mechanic phase $v_p$ and group $v_g$ velocities are often presented. I know how to derive $v_p$ and get equation: $$\scriptsize v_p=\frac{\omega}{k}.$$ What i dont ...
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### Learning about group velocity, phase velocity and particle velocity [duplicate]

I am studying quantum physics and I would like to know a bit more in detail about group velocity, particle velocity and phase velocity. Can you suggest some books/online resources where I can learn ...
2answers
79 views

### What does phase velocity physically represent, and why can it be superluminal? [duplicate]

Phase velocity is defined as $v_p=\frac{\omega}{k}$ and is described in various textbooks as being the speed at which the phase of a wave propagates. If you have a wave train that is modulated by an ...
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### Waves velocity for a standing wave of a string

I have done a laboratory experience to determine the speed of propagation of a wave in a vibrating magnetic string. To calculate the frequency (f) of oscillation I used an oscilloscope that calculated ...
1answer
43 views

### Wave packet in dispersive medium, how will the group velocity be affected? [closed]

A wave packet with center frequency ω is propagating in dispersive medium with phase velocity of 1.5 x 10^3 m/s. When the frequency ω is increased by 2%, the phase velocity is found to decrease by 3%. ...
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1answer
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### anticausal group velocity?

https://en.wikipedia.org/wiki/Anomalous_dispersion claims: Recently, it has become possible to create gases in which the group velocity is not only larger than the speed of light, but even ...
3answers
6k views

### In superluminal phase velocities, what is it that is traveling faster than light?

I understand that information cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but ...
1answer
676 views

### What is meant by 'Phase velocity is $c^2/v$' [closed]

Is phase velocity $c^2/v$ or $v/2$? Is there only one phase velocity as indicated by the above formula? If yes then why is wavepacket said to be superposition of 'waves' whose velocity is termed as ...
1answer
339 views

Why does light speed suffer a reduction when it passes through a homogeneous dielectric medium? I know my math says so, i.e.- a highly polarizable ($\chi$) medium is associated with high $\epsilon$ (=$... 1answer 467 views ### Confusion in phase and group velocity This question is based on this question asked in physics SE. In this question, they say that phase velocity greater than velocity of light. Ok, greater than velocity of light for phase velocity is ... 4answers 740 views ### Does radiation force depend on group velocity or on phase velocity? What is the radiation force$F$due to a beam of photons of power$P$undergoing perfect reflection? Is it $$a) F = 2 P / c$$ or $$b) F = 2 P / v_g$$ where$v_g$is the group velocity ? Note that ... 0answers 221 views ### Phase and group velocity of a soliton? [closed] How do I find the phase velocity and group velocity of a soliton with a$\operatorname{sech}$(hyperbolic secant) envelope? 3answers 3k views ### Two different mediums can have equal index of refraction? In other words: given a certain medium, is light's phase velocity unique? 0answers 824 views ### Wave Packets, Group velocity, and Phase velocity [duplicate] Mathematically, you find that the wave function of a particle$\Psi (x,t)$moves with the same velocity as the velocity of the particle ($v_{particle} = v_{group}$). Is there a reason why the ... 3answers 2k views ### Why is the phase velocity used in the definition of the refractive index? I'm aware of the so-called group index but why is the phase velocity used in the standard definition of the index of refraction? What advantage does this offer? 1answer 514 views ### How can we calculate (relativisticly correct) ratio$\lambda_e/\lambda_p$if proton and electron have same speeds This question is about group/phase velocities and also De Brogilie wavelength. What I would like to know is how to derive ratio$\lambda_e/\lambda_p$($\lambda_e$and$\lambda_p$are De Broglie ... 2answers 9k views ### Group Velocity and Phase Velocity of Matter Wave? In quantum mechanics, what is the difference between group velocity and phase velocity of matter wave? How can it also be that phase velocity of matter wave always exceeds the speed of light? 1answer 15k views ### Phase and Group Velocity of Electromagnetic Waves Moving charges produce oscillating electric and magnetic fields -we have an electromagnetic wave. In terms of moving charges or at the level of charges, what is phase velocity and group velocity of ... 2answers 6k views ### What does the velocity of a wave mean? I know that the velocity of a wave is given by$v=\lambda f$but what does this velocity represent in the physical sense. For instance, if I am told a car moves at a velocity of 5$m/s$I know that ... 3answers 492 views ### Controllable faster-than-light phase velocity This is not another question about faster-than-light travel or superluminal communication. I totally appreciate the speed limit capped by physical laws (or theories.) Just curious, since there is no ... 7answers 884 views ### Information relationship to Special Relativity How do we write mathematically that "information" cannot go faster than light? And along a similar line of thought, how do we relate "information" with special relativity. Lastly, what is the ... 1answer 413 views ### Wavefronts and phase velocity faster than$c\$

Lets assume we have parallel wavefronts in a glass of water: and we put an inclined rod on the water surface: related to a very small inclining, Vy velocity is greater or much greater then Vx (Vy ...