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Photon Energy Questions

  1. Jun 5, 2009 #1
    If photons always travel at the speed of light, then how do some photons have more energy than others? Wouldn't they always be traveling the same speed regardless of what angle they came in from, or how many others were coming in behind them in close proximity, or how long their wavelength was? If you have 100 photons arriving closely together, then that indicates a higher frequency, hence higher energy. But why does this matter when we talk about the energy of a single photon, since they are all travelling at the same speed?
  2. jcsd
  3. Jun 5, 2009 #2


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    The photon carries a momentum with an effective relativistic mass. The speed never changes, but its relativistic mass/momentum does change. The number of photons does not affect the frequency of the wave because that is set by the energy of the individual photons. The number of photons (which, since the speed is constant, is proportional to the rate of photons) affects the intensity of the light. The more photons, the higher the energy density of the wave but the frequency remains the same.
  4. Jun 5, 2009 #3


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    Like born2bwire says, the frequency is actually a property of the individual photons. Strange as a notion as this may seem, it's common in quantum mechanics.
  5. Jun 5, 2009 #4
    In conjunction with the above posts, the energy of a photon is E=hf.

    Photons of different energy are simply a different "color"... roygbiv in the visible spectrum for example.

    Also, you can consider that a photon emitted from a star, say our sun, has different energy as it escapes and comes towards the earth.....the frequency is redshifted due to changes in gravitational potential...it's a reflection of the work done, hence energy lost, overcoming the sun's attractive gravity...energy changes, speed doesn't.

    This is in distinct contrast to particles with mass whose speed does change with changes in kinetic energy.
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