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Speed and light

  1. Apr 11, 2003 #1
    My physics teacher says that when light enters a more dense medium it slows down, this is fine but then he states that when light leaves the same medium it again speeds up without an outside force acting on it. why is this? why does it seem to prove newton wrong?
  2. jcsd
  3. Apr 11, 2003 #2
    My understanding is that the tiny time delay is due to absorption and re-emission of photons in the medium and not an actual slowing down of the photons themselves.
  4. Apr 11, 2003 #3
    so your saying that the "apparent" increase in velocity is actually due to the refraction of the light beam?
    is there any way you could give me some example?
  5. Apr 11, 2003 #4


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    It's because the photon in a medium is doing a gigantic game of pinball, bouncing off of hundreds of thousands of atoms on its way forward. It isn't going as fast (to us) because it isn't travelling in a straight line.

    Welcome to the forums, BTW
  6. Apr 11, 2003 #5
    i realize why it slows down when it goes through the medium. where i get lost is when my teacher says that once it leaves, it again speeds up... there's no outside force acting on it and yet it speeds up. why?
  7. Apr 11, 2003 #6


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    Because it isn't speeding up. It's just going in a straight line once it leaves.

    EDIT: If you need anything more precise than that to quell your mind, I'll have to leave it to the physicists here for a better explanation.

    Last edited: Apr 11, 2003
  8. Apr 11, 2003 #7


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    Try Thinking of a photon as a car speeding down the road at 60mph. On the open road(vacuum) it travels at at a constant 60mph.

    Now it reachs a section where there are a number of stoplights (a dense medium). It still travels at 60 between each stop light, but has to pause at each light. The pauses causes its average speed through this section to drop below 60.

    once it is clear of the stoplights, its average speed returns to 60 mph.

    With light traveling through a medium it is a little different in that the light isn't stopped, but absorbed by the Atom (At this time, the photon ceases to exist) then after a short delay, another photon is emitted by the atom. which travels at c until it encounters an atom itself. Once the light has left the medium, it is free to travel without these little interuptions, and does so at its natural speed of c.
  9. Apr 11, 2003 #8
    So, wouldn't the photons be refracted into many different directions? If we shone a laser, or other concentrated light beam that could reach far into space, hypothetically, would we not be able to see it in space for the photons would be deflected into random directions and not be powerful enough for us to see it?

    Is that why a light beam will fade away after some distance, even if it isn't shown as a cone? Interesting, I completely forgot about all of that.
  10. Apr 12, 2003 #9


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    Greetings !

    I'm not exactly certain of what your question
    is in your last message, Funkee. If you're
    talking about a beam of light then it is
    comprised of many photons travelling at roughly
    the same direction. The more focused the beam
    is the longer will the photons travel relativly
    close to each other (in vacuum), but eventually
    the difference in their direction, no matter
    how small, will cause the beam to spread over
    a very large area - so that it would no longer
    be easily visible if you happen to stand in its

    The reason for the difference is the fact that
    each photon is generated by a different atom
    or separate accelerated electron and the direction
    of the emmited electromagnetic fluctuation (photon)
    is never exactly the same (not theoreticly for an
    infitesimal distance, at least).

    Does that answer it ?

    Live long and prosper.
  11. Apr 12, 2003 #10
    Yes. Essentially the question was answered in the previous posts, it was one of those crazy posts after a week of no sleep. It just makes sense, and I had never really thought about it. I just wanted to make sure I understood it.
  12. Apr 12, 2003 #11

    In a medium, right? In a vacuum, it would be a straight line right? Why would the photon's activity in relation to surrounding particles be an effect that is relative to a certain observer? isn't the effect universal?
  13. Apr 13, 2003 #12
    Theoretical explanation

    Assume the photon is a bipartite system, a positron and an electron orbiting each other around a central point moving in a straight line, held together by mutual attraction, kept apart by orbital energy which would be a function of the original emission velocity.

    Entering a medium of greater density, meaning more foreign particles per unit volume, should introduce more load on the system energy, reducing the orbital separation and deflecting the straight line path. If the planes of entry and exit are parallel, reentering the medium of lesser density should reverse the process, approximating the original path, offset and with a greater loss of energy than would have been experienced if the same distance had been traversed in the original medium.

    A wave explanation would probably be similar.

    It's an interesting notion. The orbital separation distance of the system should alter in the new medium, changing the color perceived by a viewer within the medium. The energy variation and path offset should be a function of the distance between the entry and exit surfaces. Measurements at varying distances should provide useful data. They probably already have.

    If the path of the light is visible, it is due to random reemission on contact with foreign particles, as is true of anything we see. With large numbers of individual photon systems, reemission scatters the light in all directions. The individual photon systems that remain relatively coherent comprise the offset beam that exits the medium. They are influenced by the increased force fields in the denser medium but have no direct contact with foreign particles.
  14. Apr 13, 2003 #13


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    "To us" in his post meant that we're computing net velocity, not instantaneous velocity.

  15. Apr 13, 2003 #14


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    You are exactly correct. A laser in space is invisible unless you shine it into your eye.
  16. Apr 14, 2003 #15
    It doesn't really violate Newton's laws because it's not really following them in the first place, lol. If you are talking about inertia here, that is a concept you need to throw out for light. Intertia, actually, is a measurement of mass. Light has no mass to speak of. For this reason, light always travels the same speed. The reason it "slows down" in a medium is because its progress is interfered by giant pylons (atoms) that keep pushing it off course. It still travels at light-speed, but it isn't taking the shortest route through the material, and therefore the net velocity is lower than "light-speed."

    What I never quite understood was why light slows down as it passes through glass, yet somehow goes through it in what seems like a straight line. Hope somebody here can clear that up. (Obviously, the zig-zag ping-pong game of photons we're talking about doesn't apply to the entire beam of light. I would just expect the light to disipate, though, not continue in an overall straight line.)
  17. Apr 14, 2003 #16


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    True, true, really true!!! (Hint: interferences)
  18. Apr 14, 2003 #17


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    Well, what happens is that the energy of the photons gets absorbed by electrons... the easiest direction for electrons to rerelease that energy is (via conservation of momentum) to send one or more new photons travelling the same direction as the old one. If individual atoms have very little freedom of motion, conservation of momentum virtually forces the atoms to transmit the light in the same direction it was originally going. However, in materials with a lot of freedom of motion (say... gases), the atoms have the freedom to be deflected and can send photons off in new and interesting directions.

  19. Apr 14, 2003 #18
    So if photons have no mass, how does a solar sail work?

    Let me guess, transfer of momentum via particle emission from the sun? Not photons at all?
  20. Apr 14, 2003 #19
    Thank you Hurkyl, that was very helpful. By the way, I was always curious how the momentum of light is measured, since it obviously has nothing to do with mass. Plus SR changes up the meaning of velocity too. Therefore, mv is completely out of the question.
  21. Apr 14, 2003 #20
    No Alias, it actually really does have something to do with the momentum carried by light. I have no idea how to measure the momentum of light though...as I just said lol.
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