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Speed of light confusion

  1. Oct 25, 2003 #1

    GENIERE

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    It seems many posters on these forums as well as others make a distinction between “light” and a “photon” when discussing the speed of light. Most will agree that an individual proton propagates at C whether passing through air, lead, glass or a vacuum. If one defines light as a packet of photons, or a shower of photons, it allows the possibility of saying, “the speed of light is medium dependant” while knowing that an individual photon propagates at exactly C.

    An example of the confusion caused by the distinction can be found in refraction tables developed by persons of great authority wherein they assign to light, a velocity dependant on the properties of the medium. The title of the tables usually refer to the “speed of light”. They are either ignorant or, more likely, distinguish between light and a photon.
     
  2. jcsd
  3. Oct 25, 2003 #2
    Huh?

    Most will agree that an individual proton propagates at C
    whether passing through air, lead, glass or a vacuum.


    Not me!
     
  4. Oct 25, 2003 #3

    Integral

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    Gamma Ray photons will penatatate a significant thickness of lead.
     
  5. Oct 25, 2003 #4
    Okay, which are you discussing? Protons or photons?

    Besides, penetrating a 'significant thickness of lead' is not the same as 'passing through' it.
     
    Last edited: Oct 25, 2003
  6. Oct 25, 2003 #5
    Yeah, he meant photons Chagur
     
  7. Oct 26, 2003 #6

    Integral

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    It certianly is, if you have less then that significant thickness!
     
  8. Oct 26, 2003 #7
    I really don't need the attempt at word play, Integral.

    Penetrate =/= pass through ... Check a dictionary.
     
  9. Oct 26, 2003 #8

    Integral

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    I fail to see the difference. This is physics not english.


    Edit:
    How do I look up pass through in a dictionay? If a gamma ray is able to penatrate x meters of lead ( if you wish, do some research and replace x with an actual number) and you have less then x meters of lead surrounding your gamma source there will be many gamma rays which pass through the lead. Why is this a word game?
     
    Last edited: Oct 26, 2003
  10. Oct 26, 2003 #9
    The original post, to which I replied, referred to 'Light', not 'gamma radiation'

    Sorry for not immediately bringing it to your attention and
    avoiding the futile word play you wish to engage in.

    And please don't retort with: 'But they're just different
    energy levels of the same phenomena.'
     
  11. Oct 26, 2003 #10

    Integral

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    Where does this post refer to the visible portion of the spectrum?

    edit: I must admit that I completed missed the replacement of Proton for photon. Since the entire thread is about photons, I simply read it as was intended, not as written. Of course, no proton has ever achieved the speed of light, so it makes no sense if it is read literally.
     
    Last edited: Oct 26, 2003
  12. Oct 26, 2003 #11
    Chagur, are you serious?
     
  13. Oct 26, 2003 #12
    And that is what I questioned, I thought, Integral.

    Not being that up on Physics, I was willing to grant
    that protons, like photons, could pass through air,
    glass, and a vacuum; but the lead bit didn't seem
    right and I questioned it.
     
  14. Oct 26, 2003 #13
    Yes, Bartholomew.
     
  15. Oct 26, 2003 #14

    russ_watters

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    It was a typo, Chagur. He meant "photon."

    So to clarify: a photon always travels at C. Refraction is often oversimplified to imply that it doesn't. This is especilly true in high school physics classes.
     
  16. Oct 26, 2003 #15
    Okay, russ_watters.

    Thanks.
     
  17. Oct 26, 2003 #16

    GENIERE

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    OOPS! I guess I added confusion to the confusion. Didn't mean pRoton, nor to start a little spat.
     
  18. Oct 26, 2003 #17

    Chi Meson

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    That would be SOME high school physics classes! Many textbooks (Paul Hewitt's especially) try to make it clear that light waves/photons always travel at c (disregarding the latest theories of variances).

    The net propagation of light is slowed by the photons interactions with the atoms and molecules of the substance it is passing through. But while the photon is travelling from atom to atom, it is propagating at c.
     
  19. Oct 29, 2003 #18
    Chagur, "penetrate a certain distance" of a material is the same as "penetrate through" the material, if the material isn't as thick as the distance. It doesn't matter whether the thing doing the penetrating is a photon, a proton, or a bullet.

    You accept that light can pass through glass. It certainly wouldn't pass all the way through 10 miles of smoked glass. It would penetrate only to a certain distance. But if you reduced the thickness of the glass, eventually light would start coming through in significant amounts--it would start penetrating all the way through.

    The same thing is true for lead, on a smaller scale. Instead of feet of glass that the photon can pass through, you have fractions of a millimeter of lead. The light will penetrate that far, and if you can hammer out the lead thinner than that fraction of a millimeter, the lead will be transparent.
     
  20. Nov 2, 2003 #19
    "But if you reduced the thickness of the glass, eventually light would start coming through in significant amounts--it would start penetrating all the way through."

    When you say "eventually", are you referencing time or thickness??? I first read it as time, but assume thickness. Right???

    Nautica
     
  21. Nov 3, 2003 #20
    Are we talking about a photon, or about wave theory?
    Are the photons electron free?

    Please define "light".
     
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