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A little confused

  1. Oct 7, 2005 #1
    if photon is light considered as particles, then shouldn't it always have the speed of light? but sometimes i see that it doesn't have the speed of light in my textbook...
     
  2. jcsd
  3. Oct 7, 2005 #2

    Physics Monkey

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    Could you be more specific about your question i.e. what book and in what context? In free space, light does travel at the free space speed of light. I think perhaps you are referring to the speed of light in a medium which can be less than the free space speed of light. Is this right?
     
    Last edited: Oct 7, 2005
  4. Oct 7, 2005 #3
    the book is concepts of modern physics by arthur beiser... the context is talking about light and relativity~
    may i ask: what is meant by "the free space"?
    @@a
     
  5. Oct 7, 2005 #4

    Physics Monkey

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    Free space basically means vacuum. Light can move slower than its speed in vacuum when it is in a medium like water or glass, and this is described by the index of refraction of the medium. Since it is a book on "relativity", perhaps the author is speaking of some gravitational effect on the motion of light? Could you provide a specific example?
     
  6. Oct 7, 2005 #5

    jtbell

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    Which page? Fifth or sixth edition? (I think I have both)
     
  7. Oct 7, 2005 #6
    if you have the 6th edition, on pg 82, you'll find an example 2.6~
    for the last part of the question, my reasoning (might not be correct) is that since the two photons have different energy, then they should have different speeds?
     
  8. Oct 7, 2005 #7

    ZapperZ

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    If this came from a chapter or topic on Relativity, maybe you should go back and reread the postulates of special relativity that Einstein established. With that, you'll see that one of the postulate will not be compatible with your reasoning that "...since the two photons have different energy, then they should have different speeds....".

    Zz.
     
  9. Oct 7, 2005 #8
    opps~ you're right...
     
  10. Oct 7, 2005 #9

    jtbell

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    OK, found it, thanks!

    I was going to elaborate on Zz's answer, but I can't seem to post equations at the moment. The LaTeX engine seems to be having problems as a result of the forum software upgrade last night. Suffice it to say that the momentum and energy of a photon are both independent of its speed (since all photons travel at the same speed). The momentum and energy are related by a factor of c: E = pc, which is a special case of Beiser's equation (1.24) on page 31, with m = 0.
     
  11. Oct 8, 2005 #10
    lol~
    it seems that everything is strange in modern physics...
    thanks!
    however, if a photon's momentum can't be calulated by mv, what is the physical meaning of momentum for a photon?
     
  12. Oct 8, 2005 #11

    Astronuc

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  13. Oct 8, 2005 #12

    jtbell

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    It can give momentum to another particle, and receive momentum from another particle.

    Note that in classical electromagnetism, the electric and magnetic fields also carry momentum and energy. In fact, the electromagnetic field energy and momentum in an electromagnetic wave are related by the same equation as for photons in relativity: E = pc
     
  14. Oct 8, 2005 #13
    so you mean that the physical meaning of the momentum of a photon is still the degree to which a photon can change its motion?
     
  15. Oct 8, 2005 #14

    Gokul43201

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    That is not the physical meaning of the momentum of anything...let alone the photon. Did you mean "change something else's motion" ?
     
  16. Oct 9, 2005 #15
    um, yes, that's what i meant~
    :P
     
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