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Phase velocity of EM radiation in free space

  1. May 16, 2013 #1
    1. The problem statement, all variables and given/known data

    (i) What is the phase velocity of EM radiation in free space?

    (ii) What is its group velocity?


    2. Relevant equations



    3. The attempt at a solution

    Using the definition as per this page:
    http://physicsdaily.com/physics/Phase_velocity

    (i) Vp = (C^2)/Vg

    (ii) Vg = (C^2)/Vp
     
  2. jcsd
  3. May 17, 2013 #2

    Andrew Mason

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    They are the same: c

    AM
     
  4. May 17, 2013 #3
    So is the eqn which I have referred to, and used for my solution, the incorrect one to use on this occasion?

    Thanks
     
  5. May 18, 2013 #4

    Andrew Mason

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    The equation is correct: VpVg = c^2 ; Vp=Vg=c

    AM
     
  6. May 18, 2013 #5
    Thank you.
     
  7. May 19, 2013 #6
    Since, as you have above Vp=Vg=c, does that mean I can further reduce the two solutions I had in my original post from

    (i) Vp = (C^2)/Vg

    (ii) Vg = (C^2)/Vp

    to

    (i) Vp = c

    (ii) Vg = c
     
  8. May 19, 2013 #7

    Andrew Mason

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    You cannot conclude that Vp = c from the equation: VpVg=c2. You would have to know that the phase velocity of light in a vaccum is equal to the group velocity.

    A difference between phase velocity and group velocity occurs if the speed of a wave is frequency dependent. But the speed of em waves in a vacuum do not depend on frequency.

    AM
     
  9. May 19, 2013 #8
    Yes, of course. That's a silly mistake to have made.

    Thanks again.
     
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