Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    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]

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

    jtbell

    User Avatar

    Staff: Mentor

    (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

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: E=hf question about mass
  1. Derivation of E=hf? (Replies: 8)

Loading...