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Why photons have momentum?

  1. Mar 12, 2012 #1
    p=mv then for a photon p=0*c means p=0. And also ke=1/2pv which is ke=0 and E=mc2 means E=0. please use easy words and we descriptive.
  2. jcsd
  3. Mar 12, 2012 #2


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    For <particles> of light, photons, certain care needs to be taken about what p,m,v,E mean. If p=mc, then m is the so-called 'relativistic mass' which has no meaning for a photon, but only for a massive particle.

    The universal formula which is to be used is

    [tex] E^2 = \vec{p}^2 c^2 + m^2 c^4 [/tex]

    where m is the rest/invariant mass which is 0 for a photon.
  4. Mar 12, 2012 #3
    See the FAQ here. Also, you might find this thread interesting.
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