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Question about Light

  1. Jul 10, 2008 #1
    Ok, so light is both a wave and a particle. That's fine, I accept that.
    If a photon is a particle it must therefore have a mass.
    But, Einstein's theory of relativity states that material bodies cannot travel faster than the speed of light.
    As a particle approaches the speed of light it becomes heavier i.e. its mass increases.
    If a particle is travelling at the speed of light, its mass is infinite. It would require an infinite force to move it.

    So.....how does this account for light being a particle-the photon?
  2. jcsd
  3. Jul 10, 2008 #2


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    That depends on which kind of mass you're talking about: rest mass or relativistic mass.

    Photons have zero rest mass. The general relationship between energy, momentum and rest mass is [itex]E^2 = (pc)^2 + (m_0 c^2)^2[/itex]. For a photon this reduces to E = pc.

    Of course, "rest mass" sounds kind of silly when we're talking about photons, which can never be at rest. That's why many physicists use the term "invariant mass" instead.

    The usual formula for relativistic mass doesn't work for photons because it reduces to 0/0 which is undefined mathematically. Instead you can say the relativistic mass of a photon is given by [itex]E = mc^2[/itex], that is, [itex]m = E/c^2[/itex].
  4. Jul 10, 2008 #3


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    Where do you draw this conclusion from?
  5. Jul 10, 2008 #4


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    Staff: Mentor

    And who says light is a particle anyway? It behaves, at times, like a particle or wave, but light is, in fact, not a particle or a wave. Light is something else altogether.
  6. Jul 18, 2008 #5
    i thought light was a particle whose movement is determined by EM waves?
  7. Jul 18, 2008 #6
    Many say that the photon has no mass!! As I see on the Forum that gives place to confusions.
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