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Energy of photon never becomes zero

  1. Oct 23, 2011 #1
    as we know that light consist packets of energy called photons
    And mass of a
    body when moving with the speed of light increases so, the mass of photon increases when it moves with light and when mass increases its energy is also increasing it means that when light travels it never stops and continousely gain energy
    But in reality intensity of light at certian distance becomes zero...why?
  2. jcsd
  3. Oct 23, 2011 #2


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    An object with (rest) mass never moves at the speed of light relative to any inertial observer. It gains energy and speed by forces acting on it. You can treat <total energy>/c^2 as relativistic mass, but this often just leads to confusion (there is only one Newtonian formula you can plug it into and get the right result).

    A photon never changes energy or speed relative to an inertial observer. However, its energy (but not speed) are observer dependent due to the Doppler effect.

    The decrease in intensity of light with distance is unrelated to photon's energy, it is a function of the number of photons per unit area. If you had a perfectly collimated ideal laser, its intensity would not change with distance. If you have N photons/second emitted in a spherical pattern, then as you double distance you have 1/4 as many photons per unit area.
  4. Oct 23, 2011 #3


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    None of this is correct. A photon has whatever energy it has (proportional to its frequency) and its (invariant) mass is always zero. It does not gain invariant mass nor relativistic mass nor energy as it travels.
  5. Oct 23, 2011 #4
    The mass m of an object is invariant. You seem to refer to the mass M that appears in very old textbooks and in some 'popular' presentations of relativity. In any case that M is only defined for massive particles, not for photons.

    A photon is a massless particle m=0. Precisely being massless is the reason that its speed is constant and equals c.
  6. Oct 24, 2011 #5
    Dear it may be due to my little knowledge but if u try to explain more pricisely step wise that how mass of moving proton not increase so i am able to understood
  7. Oct 24, 2011 #6
    I would say that if the mass of the proton is equal to zero and you follow the basic rules of multiplication the answer is there. However i believe you ask the question under the assumption that the photon must have mass. E is equal to the mass times the speed of light squared would also mean that the photon has no energy to work backwards. I may be completly wrong and probably am
  8. Oct 24, 2011 #7


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    Do you mean proton or photon? I'll answer both:

    1) If a proton is accelerated, its energy increases. You can choose to consider this (increasing) E/c^2 as increasing relativistic mass - but this leads to confusion because you can't use in place of mass most formulas of Newtonian mechanics. Thus, my preference is just to see it as increasing energy, while rest mass remains constant.

    2) A photon cannot be accelerated and cannot change energy relative to a given inertial observer. Different inertial observers can see a given photon having different energies. It is very misleading to view a photons 'relativistic mass' as E/c^2 - you will almost always be led to wrong conclusions doing this.

    3) I explained already how light intensity decreasing with distance is related only to number of photons per unit area.
  9. Oct 24, 2011 #8
    Could you explain your statement wrt gravitational red- and blueshift?
  10. Oct 24, 2011 #9


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    The context was pure SR. I saw no need to add confusion; there was enough already.
  11. Oct 25, 2011 #10
    Sorry for confusing u i wrote proton instead of photon
  12. Oct 25, 2011 #11


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    You have a very faulty understanding of Special Relativity. Please read the FAQ subforums, both in the General Physics forum and the Relativity forum.

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