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E=mc2 e=hf

  1. Mar 21, 2013 #1
    e =mc2. e = hf

    is it different????
    than why??

    ..e =mc2 this only use in nuclear??
  2. jcsd
  3. Mar 21, 2013 #2
    E=mc^2 is the rest mass of a massive particle.
    That other thing over there is the energy of a photon.
  4. Mar 21, 2013 #3
    If you put the two together and write f=c/λ then you can rearrange
    mc2 = hc/λ to give λ = h/mc
    DeBroglie did precisely this to come up with his hypothesis that λ = h/momentum
    Which is the basis of wave - particle duality
  5. Mar 21, 2013 #4


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    The equation E=MC2 is used to find the energy content of some amount of mass. (Rest mass/invariant mass)

    The equation E=hf is used to find the energy content a photon, where f = frequency of the photon.

    Note that E=MC2 is actually part of a larger equation: E2=M2C4+P2C2, where P = momentum of the object. This allows you to find the energy content of an object based on its mass AND its motion relative to yourself.
  6. Mar 22, 2013 #5


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    as Emilyjoint is saying, if we take 'm' to be the 'relativistic mass', then e =mc2 and e = hf are both correct for a photon.

    But as Drakkith is saying, if we take 'm' to be the 'rest mass', then the full equation is E2=M2C4+P2C2 and for a photon, the rest mass is zero, so E=PC for a photon.
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