Relation between Group velocity and Phase velocity

  • Thread starter neelakash
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Is there any unconventional method to find out the relation between group velocity and phase velocity? know there is a method employing Fourier tarnsforms and another easier method as well
 

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  • #2
Meir Achuz
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Phase and group velocities only have meaning using a Fourier transform.
w/k and dk/dw are in terms of Fourier variables.
 
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one thing to notice is that, for a group velocity to be meaningful, the wave packet has to be merely composed of wavenumber in a narrow range.
 
  • #4
Meir Achuz
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one thing to notice is that, for a group velocity to be meaningful, the wave packet has to be merely composed of wavenumber in a narrow range.
That is a very good point. The equation v_g=dk/dw comes from keeping only the first term of a Taylor expansion. Many of the papers claiming funny group velocities for light miss the point you mention.
 

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