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Thread: How does light know when to Reflect, Refract, or Pass Through matter?

  1. #1 How does light know when to Reflect, Refract, or Pass Through matter? 
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    I've learned over the years that photons are absorbed and re-emitted when they interact with matter. We know that things that are polished can reflect light. Things that are clear can refract and let light pass through, but at the scale of atoms, how does the photon know which path to take?


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    It depends on which photons and which material it is aimed at.

    Light, in itself isn't just a particle, it is a wave as well, (depending on which interpretation you want to follow). This wave makes it so that this light isn't simply touching an atom, which reabsorbs it, and then reemits it. It touches the interaction between molecules. These covalent/ionic/other bond more readily share electrons, and this sometimes has the effect that is reflect or bend light. It all depends on which bonds, and which atoms they consist of at to which wavelength of light they reflect, transmit or absorb.

    Then there are quirks like a maximum reflective angle etc..

    I really have no idea how to cover this with formulas. I don't even know if science really understand it perfectly. But i do know..

    - Light knows nothing...
    - A mirror does not absorb and then re-emit light...
    - Any surface, no matter how smooth, is actually not perfectly smooth on a molecular level.


    Growing up, i marveled at star-trek's science, and ignored the perfect society. Now, i try to ignore their science, and marvel at the society.

    Imagine, being able to create matter out of thin air, and not coming up with using drones for boarding hostile ships. Or using drones to defend your own ship. Heck, using drones to block energy attacks, counterattack or for surveillance. Unless, of course, they are nano-machines in your blood, which is a billion times more complex..
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    Thanks for the explanation Zwolver. It sounds like there's some missing pieces to this though. The furthest I went on the subject involved photon interactions within material structures, but nothing at the atomic level.
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  5. #4  
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    Quote Originally Posted by Metallicbeing View Post
    Thanks for the explanation Zwolver. It sounds like there's some missing pieces to this though. The furthest I went on the subject involved photon interactions within material structures, but nothing at the atomic level.
    This is quite an involved and interesting subject. Refraction involves a coupling of the electric vector of the photon wave to the structure of the matter it passes through. It polarises the material, causing the electrons to move in synch with the field and this affects the phase velocity of the wave as it passes (imagine trying to walk on a trampoline that moves with your feet). But the photons are NOT absorbed. If they were, the material would be opaque at that wavelength.

    In fact, something interesting happens at wavelengths that are close to an absorption frequency for the material. The polarisability of the medium increases as the wavelength gets closer to that of the absorption frequency, and this causes the refractive index to be a function of wavelength. This is why one gets "dispersion", for example in a prism, due to the wavelength dependence of the refractive index.
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    Here's another explanation from Professor Merrifield.

    https://www.youtube.com/watch?v=CiHN0ZWE5bk
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    Thanks for the explanations and video. They were informative.

    It seems that photons go through a series of Collective (material structure) and Apparent (quantum state) processes before you get the end result of Transparency, Refraction, and Absorption. Out of all of this, I think Transparency and Absorption were the easiest to understand. If the photon doesn't have enough energy to raise an electron to its next higher energy state, then the photon passes through the material and it will seem to be transparent or some variation of transparency. If the photon does have enough energy to bring an electron to its next energy level, then the photon is absorbed. I think I understand Refraction as a reduction of the speed of light through the material. The more reduction of the speed of light, the more Refraction there will be. Please let me know if I understood correctly. Thanks.
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    Quote Originally Posted by Metallicbeing View Post
    I think I understand Refraction as a reduction of the speed of light through the material. The more reduction of the speed of light, the more Refraction there will be. Please let me know if I understood correctly. Thanks.
    Well, sort of, but no, not really.

    The phase velocity is different but the wave is still moving at the speed of light.

    "The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves."—Wiki
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    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by Metallicbeing View Post
    I think I understand Refraction as a reduction of the speed of light through the material. The more reduction of the speed of light, the more Refraction there will be. Please let me know if I understood correctly. Thanks.
    Well, sort of, but no, not really.

    The phase velocity is different but the wave is still moving at the speed of light.

    "The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves."—Wiki
    By Phase Velocity, do you mean the photon is changing color or is just incoherent?
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    Quote Originally Posted by Metallicbeing View Post
    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by Metallicbeing View Post
    I think I understand Refraction as a reduction of the speed of light through the material. The more reduction of the speed of light, the more Refraction there will be. Please let me know if I understood correctly. Thanks.
    Well, sort of, but no, not really.

    The phase velocity is different but the wave is still moving at the speed of light.

    "The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves."—Wiki
    By Phase Velocity, do you mean the photon is changing color or is just incoherent?
    Try this. If this doesn't help you understand it, there's tons of videos and lectures out there on group and phase velocity.

    https://www.youtube.com/watch?v=CtFS...tu.be&t=41m13s
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  11. #10  
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    Pity the "Like" button does not work. This is a good discussion.

    There is an animation on Wiki showing one possible relation between Group and Phase velocities here: https://en.wikipedia.org/wiki/Group_velocity

    Unfortunately it seems to have been changed, so that instead of showing light with a group velocity faster than the phase velocity (which is what we have in an optically dense medium) it now shows a wave in which the Group velocity is slower than the Phase velocity. Nonetheless it does still illustrate the principle that the envelope of the disturbance can travel at a different rate from the the frequency of the waves that comprise it.

    I suppose (a thought that has only now just occurred to me, durrh) this is like the principle of amplitude modulation in radio signals.
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    Quote Originally Posted by exchemist View Post
    Pity the "Like" button does not work. This is a good discussion.
    Really? We’ve had this same exact discussion before. You should have posted those videos yourself.

    You know, on a few occasions, I thought—hmm—maybe he’s just testing me. I mean, it’s not like I haven’t done the same thing before myself, but then we had this one discussion that left me wondering if you were even an exchemist. I provided you with a link but you didn’t even read it.

    Aha so that's it. Some sort of accumulation of carbon compounds, due to reaction between the newly exposed metal surface and ambient CO2. Good, thanks for that. And no I had not read the Japanese paper, as you seemed to know all about it and I had hoped you might tell me what you had found out.—Exchemist
    Japanese paper? You mean JAP as in the "Journal of Applied Physics"?—Secular Sanity
    Oh yeah…durrrh!—Exchemist
    Could be your age, memory, lack of interest, or just pure laziness. Let’s just hope that you’re not a faker fox, eh?

    I’m not judgin'—I’m just sayin'

    Just keeping it real, you know.

    Good day to you!
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  13. #12  
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    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by exchemist View Post
    Pity the "Like" button does not work. This is a good discussion.
    Really? We’ve had this same exact discussion before. You should have posted those videos yourself.

    You know, on a few occasions, I thought—hmm—maybe he’s just testing me. I mean, it’s not like I haven’t done the same thing before myself, but then we had this one discussion that left me wondering if you were even an exchemist. I provided you with a link but you didn’t even read it.

    Aha so that's it. Some sort of accumulation of carbon compounds, due to reaction between the newly exposed metal surface and ambient CO2. Good, thanks for that. And no I had not read the Japanese paper, as you seemed to know all about it and I had hoped you might tell me what you had found out.—Exchemist
    Japanese paper? You mean JAP as in the "Journal of Applied Physics"?—Secular Sanity
    Oh yeah…durrrh!—Exchemist
    Could be your age, memory, lack of interest, or just pure laziness. Let’s just hope that you’re not a faker fox, eh?

    I’m not judgin'—I’m just sayin'

    Just keeping it real, you know.

    Good day to you!
    Well my memory is not too good for what I've posted on forums, so you may be right. I don't recall you and I discussing light refraction etc, but we could well have - it comes up from time to time.

    But I almost never post videos. I nearly always find them slow to get to the point and either glib or unclear, for explaining anything of any subtlety or complexity.

    I'll let you work out whether I am a faker fox by what I post over time. The only other science contribution I have made since rejoining yesterday is under Chemistry. (PhDemon and I found out once long ago we had not only attended the same university but that my tutor was his research supervisor and the lecturer I found most inspiring was his tutor! And our ages are 20 years apart.....).
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    Quote Originally Posted by exchemist View Post
    Well my memory is not too good for what I've posted on forums, so you may be right. I don't recall you and I discussing light refraction etc, but we could well have - it comes up from time to time.

    But I almost never post videos. I nearly always find them slow to get to the point and either glib or unclear, for explaining anything of any subtlety or complexity.

    I'll let you work out whether I am a faker fox by what I post over time. The only other science contribution I have made since rejoining yesterday is under Chemistry. (PhDemon and I found out once long ago we had not only attended the same university but that my tutor was his research supervisor and the lecturer I found most inspiring was his tutor! And our ages are 20 years apart.....).
    We also learned you were a crusty old sod (and got your supplies in the Co-op)
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    Quote Originally Posted by exchemist View Post
    Well my memory is not too good for what I've posted on forums, so you may be right. I don't recall you and I discussing light refraction etc, but we could well have - it comes up from time to time.

    But I almost never post videos. I nearly always find them slow to get to the point and either glib or unclear, for explaining anything of any subtlety or complexity.

    I'll let you work out whether I am a faker fox by what I post over time. The only other science contribution I have made since rejoining yesterday is under Chemistry. (PhDemon and I found out once long ago we had not only attended the same university but that my tutor was his research supervisor and the lecturer I found most inspiring was his tutor! And our ages are 20 years apart.....).
    An honest answer. I appreciate that. They’re rare. Thanks!

    Well, Metallicbeing may find another little tidbit about dispersion interesting. It’s a silly little experiment that s/he can try at home.

    Not only can metamaterial reverse the out-going spectrum, but if you look at the light source through a prism, the spectrum is also reversed. You can understand why with a little ray tracing. Your eyes are tracing the rays through the prism to a virtual image behind it. The lines cross at the prism, making it appear as if the red has exited towards to base and the blue towards the apex.

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    Quote Originally Posted by geordief View Post
    Quote Originally Posted by exchemist View Post
    Well my memory is not too good for what I've posted on forums, so you may be right. I don't recall you and I discussing light refraction etc, but we could well have - it comes up from time to time.

    But I almost never post videos. I nearly always find them slow to get to the point and either glib or unclear, for explaining anything of any subtlety or complexity.

    I'll let you work out whether I am a faker fox by what I post over time. The only other science contribution I have made since rejoining yesterday is under Chemistry. (PhDemon and I found out once long ago we had not only attended the same university but that my tutor was his research supervisor and the lecturer I found most inspiring was his tutor! And our ages are 20 years apart.....).
    We also learned you were a crusty old sod (and got your supplies in the Co-op)
    Well you're right about the first part. But I hardly ever shop at the Co op - that bloody music! I'm more of a Sainsbury's man. But not Waitrose.
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    I thought you told me you shopped for the best bread at the Co-op and it was a crusty (white) loaf.
    Unless you were being a faker fox
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  18. #17  
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    Quote Originally Posted by geordief View Post
    I thought you told me you shopped for the best bread at the Co-op and it was a crusty (white) loaf.
    Unless you were being a faker fox
    Ah no, that was Woolworth's. The "Cadena Bakery", in Woolies in Cornmarket, now long gone.
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by geordief View Post
    I thought you told me you shopped for the best bread at the Co-op and it was a crusty (white) loaf.
    Unless you were being a faker fox
    Ah no, that was Woolworth's. The "Cadena Bakery", in Woolies in Cornmarket, now long gone.
    My mistake (I take it all back)
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    Thank you for helping me understand Phase Velocity, but I'm still not clear how that affects Refraction (I'm picturing the resultant waveform over a duration of time as the phase shifts).
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    Quote Originally Posted by Secular Sanity View Post
    Well, Metallicbeing may find another little tidbit about dispersion interesting. It’s a silly little experiment that s/he can try at home.

    Not only can metamaterial reverse the out-going spectrum, but if you look at the light source through a prism, the spectrum is also reversed. You can understand why with a little ray tracing. Your eyes are tracing the rays through the prism to a virtual image behind it. The lines cross at the prism, making it appear as if the red has exited towards to base and the blue towards the apex.

    This meta-material, is it the same as a Sun Stone?
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  22. #21  
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    Quote Originally Posted by Metallicbeing View Post
    Thank you for helping me understand Phase Velocity, but I'm still not clear how that affects Refraction (I'm picturing the resultant waveform over a duration of time as the phase shifts).
    That would be because when a wave strikes the interface with an optically denser medium at an oblique angle, the side that hits it first starts to propagate more slowly while the rest only does the same progressively, as it intersects the interface. This causes the wave to change direction. Diagram here: https://voer.edu.vn/file/53789
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    Quote Originally Posted by Metallicbeing View Post
    This meta-material, is it the same as a Sun Stone?
    Google metamaterial and then you tell me.

    Some metamaterial can reverse the out-going spectrum, but if you look through a simple cheap prism, you’ll notice the spectrum of the light source itself (a candle works well) is reversed because your eyes see it in the direction that the light enters them from the object. The red is least bent, but appears to be most bent, and the violet appears to be the least bent.
    Last edited by Secular Sanity; August 3rd, 2017 at 11:07 AM.
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    It turns out the Sun Stone (a.k.a. Iceland Spar or Calcite) just polarizes light. So, it's not a metamaterial. I did find out they hope to create some type of cloaking technology with metamaterial though; cool.
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