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Does Light is massless ?

  1. Oct 10, 2004 #1
    Does light is massless ?
  2. jcsd
  3. Oct 10, 2004 #2
  4. Oct 10, 2004 #3


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    Light has momentum, not mass.
  5. Oct 10, 2004 #4
    Maybe, you know the relationship between relativistic and inertial mass, that is:

    [tex]M_{rel} = \gamma M_{inertial}[/tex], where [tex]\gamma[/tex] is:

    [tex]\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2} }}[/tex]

    You should see here that as [tex] v \rightarrow c[/tex], [tex]\gamma \rightarrow \infty[/tex]

    So, to avoid the concept of infinite mass, we must cheat here. We say that any kind of particle which speed is c, can not have inertial mass.
  6. Oct 10, 2004 #5


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    yeah, but a photon has energy. And energy is mass (E=mc2). So a photon traveling at c, has mass.
    And how can a photon have a momentum without mass??
  7. Oct 10, 2004 #6


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    A photon has energy E=pc.
    Generally, E 2 =(mc2)2+(pc)2.

    You may recall from QM that [tex]p=h/\lambda[/tex].
  8. Oct 10, 2004 #7


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    [tex]E = m c^2[/tex] is used to explain the maximum potential engery of a chunk of mass. For example if the mass where annihilated and coverted into energy. It's not meant to imply that engery has mass.

    Photons only travel at the speed of light in a vacuum. They slow down when traveling through lenses or prisms for example, at least at the macro level. At the atomic level, I'm not sure if photons bounce off electrons, or if they're absorbed and then re-emitted by electrons. If they're absorbed and re-emitted, then maybe they alway travel at the speed of light and it's the absortion and re-emission process that slows the effective speed down.
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