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I Plasma self-focusing index of refraction < 1?

  1. Apr 24, 2017 #1
    Hi. After reading about self-focusing in plasmas I came across the relative index of refraction that a light wave experiences during the interaction with plasma. Namely [tex]n = \sqrt{1-\frac{{\omega_p}^2}{{\omega}^2}}[/tex] where [tex]\omega_p[/tex] is the plasma frequency and [tex]\omega[/tex] is the light (for example laser) frequency. Clearly this is smaller than 1 but how can that be? What does it imply? What's the speed of light due to this self-focusing?
  2. jcsd
  3. Apr 28, 2017 #2


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    This just means that the phase velocity of electromagnetic waves in the plasma is faster than c (just like for EM waves in a waveguide) However, the group velocity is smaller than c so there is no violation of special relativity.
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