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Thread: Why does light slow down in water?

  1. #1 Why does light slow down in water? 
    KJW
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    https://www.youtube.com/watch?v=CUjt36SD3h8

    This YouTube video gives two wrong answers (which it explains are wrong) and the correct answer.

    One of the wrong answers, the absorption-emission answer, is so commonly given that I felt the need to post this thread.


    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Refraction?


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    Quote Originally Posted by KJW View Post
    https://www.youtube.com/watch?v=CUjt36SD3h8

    This YouTube video gives two wrong answers (which it explains are wrong) and the correct answer.

    One of the wrong answers, the absorption-emission answer, is so commonly given that I felt the need to post this thread.
    A nice explanation that avoids the pitfalls of the common pseudo-explanations. He doesn't get into the phase/signal/group velocity stuff of course, but as its aimed as a young audience that's fair enough.
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    Quote Originally Posted by mathman View Post
    Refraction?
    That is explained by the change in phase velocity, yes.
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    it's slowed by the matter....
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    I think it's because of diffraction of light meeting H20 molecules that lengther the path of light
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    Quote Originally Posted by manoir View Post
    I think it's because of diffraction of light meeting H20 molecules that lengther the path of light
    The lengthening of the light path due to scattering was the other wrong answer given in the YouTube video.

    That the correct answer is indeed correct can be established by considering the property of the medium that determines the refractive index. This property is the polarisability of the electron density of the medium in response to the rapidly oscillating electromagnetic field. For example, benzene with its relatively polarisable pi-electrons has a somewhat higher refractive index than saturated hydrocarbons (giving it a rather peculiar appearance compared to other solvents used by organic chemists). It should be noted that in a hypothetical medium consisting of perfectly rigid charges, there would be no refraction.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    I remember watching a vsauce video where Michael Stevens (the host) said that light does not slow down at all, but it simply takes a convoluted path, with curves that are too small to see (I hope that makes sense; I think it matches with what he said the first wrong answer was).
    It gets confusing when you hear one thing, and then someone else goes "actually, ..." and then later you find out that the "actually, ..." wasn't true, and then possibly more things after that. That's why I find it nice to be on a forum with people who actually studied these things, where there wasn't a problem of over-simplifying things to make it accessible to the layman.
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    Quote Originally Posted by anticorncob28 View Post
    I remember watching a vsauce video where Michael Stevens (the host) said that light does not slow down at all, but it simply takes a convoluted path, with curves that are too small to see (I hope that makes sense; I think it matches with what he said the first wrong answer was).
    It gets confusing when you hear one thing, and then someone else goes "actually, ..." and then later you find out that the "actually, ..." wasn't true, and then possibly more things after that. That's why I find it nice to be on a forum with people who actually studied these things, where there wasn't a problem of over-simplifying things to make it accessible to the layman.
    The oscillating electric field of the light polarises the medium so that the electrons in the medium move back and forth move a bit out of synch and the whole thing becomes a bit like trying to walk on a trampoline. This slows down the phase velocity of the light.
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    if i understand well it s due to interference and not diffraction. The moving of the electron create another electromagnetic wave that interfere with the incident one. But for me it's not clear why it will change the incident angle or the velocity of the light. Because the electrons will oscillate at a certain frequency but in void the light velocity do not depend of the frequency. So why does it have an importance here still thinking about it. Anyway i don t think the path of light in matter is straight on, i don t imagine it like that.
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    Quote Originally Posted by manoir View Post
    if i understand well it s due to interference and not diffraction. The moving of the electron create another electromagnetic wave that interfere with the incident one. But for me it's not clear why it will change the incident angle or the velocity of the light. Because the electrons will oscillate at a certain frequency but in void the light velocity do not depend of the frequency. So why does it have an importance here still thinking about it. Anyway i don t think the path of light in matter is straight on, i don t imagine it like that.
    Agree it is far from straight forward. You need to distinguish between phase velocity, which is what is reduced in a medium and determines angle of refraction etc from group velocity. My understanding is that in most cases signal velocity =group velocity and this remains at c. But a real physicist may be able to comment more authoritatively than I can.
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    Another interesting question if we consider light as corpuscle and not wave, why its velocity is change ? Is it possible to explain it in this term (as a corpuscule) ?
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    Quote Originally Posted by manoir View Post
    Another interesting question if we consider light as corpuscle and not wave, why its velocity is change ? Is it possible to explain it in this term (as a corpuscule) ?
    I've given this question considerable thought even before you asked it, and came to the conclusion that it is not possible to explain refraction in terms of particles for the same reason that it is not possible to explain diffraction and interference in terms of particles... these are wave phenomena. Alternatively, once the electromagnetic wave itself is explained in terms of particles, then all the wave phenomena, including refraction, can be explained in terms of particles.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    It should be linked to hazard and the fact that you do not have only one path, in my opinion
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