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E=hf question about mass

  1. Apr 22, 2013 #1
    Ok so the equation E= hf about the energy of a photon, I'm having a problem understanding energies to do with photons.
    Since, E=hf
    ∴ 1/2mv2=hf. But if m=0 how can a photon have energy?
  2. jcsd
  3. Apr 22, 2013 #2


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    hf is NOT the "kinetic energy" of photons. So you cannot equate things that are different.

    Read this FAQ Thread.

    https://www.physicsforums.com/showthread.php?t=511175 [Broken]

    Last edited by a moderator: May 6, 2017
  4. Apr 22, 2013 #3


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    (1/2)mv^2 doesn't work for energy in relativity. The general relationship between energy, mass and momentum in relativity is

    ##E^2 = (mc^2)^2 + (pc)^2##

    where m is the invariant mass which is sometimes called "rest mass." For photons, m = 0 so E = pc, that is, they can have both energy and momentum even though they don't have ("rest") mass.
  5. Apr 22, 2013 #4
    But if p=mv then are we not back at the same problem, sorry for my ignorance I just wish to understand it.
  6. Apr 22, 2013 #5
    The point is that ##p=mv## does not hold for a massless object. If it would, then ##v=\infty## and we have nonsense. I recommend Giancoli's 6th Edition of Physics for a deeper understanding of Photons.
  7. Apr 22, 2013 #6


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    Callum, p=mv is valid in Newtonian physics only (in the form [itex] \displaystyle{\vec{p}=m\vec{v}} [/itex]). You may have seen p=mv with m the so-called <relativistic mass> which is a highly useless and missleading notion, now abandoned even in introductory texts.
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