Explaining transparence

Discussion in 'Physics & Math' started by Magical Realist, Oct 16, 2016.

  1. Magical Realist Valued Senior Member

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    I've asked this before. This time I want it in plain english. How is it possible for light to be transmitted thru transparent matter? Don't the photons hit the atoms of the glass or plastic? Or are they bent around them? How?
     
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  3. PhysBang Valued Senior Member

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  5. Magical Realist Valued Senior Member

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    Crystal clear. Is this also the reason water is transparent? That the energy gap of water atoms is too much for the light to push the electrons to a higher state? I'm thinking maybe here the case is rather that the atoms are more spread apart than in a solid. That suggests that every opaque liquid is basically a suspension of clear liquid with particles floating in it. Except maybe for mercury. That may be a case of a pure opaque liquid.
     
    Last edited: Oct 16, 2016
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  7. exchemist Valued Senior Member

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    Er, what point is being made here?
     
  8. Confused2 Registered Senior Member

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    Maybe the next question is going to be "Why does light travel more slowly through glass than a vacuum?" - if it doesn't interact then what is it 'seeing'?
     
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  9. Fednis48 Registered Senior Member

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    The quantum mechanics of light travelling through bulk matter is surprisingly subtle, and there's a lot of wrong information (or at least misleadingly-presented information) out there. In my experience, one of the biggest problems is that people tend to think of the light/matter interaction only in terms of discrete excitations. The intuition is that light travelling through matter can get absorbed by atoms (or molecules), causing the atoms to excite. At some later time, the atoms decay back down to the ground state, emitting light that's the same color as the absorbed light but in a "scattered" pattern. This is an excellent description for the elemental absorption/emission lines used in spectroscopy, and it's a decent description for the extreme cases of highly transparent (e.g. air) or highly opaque (e.g. paint) substances. In the intermediate range, though, it's an oversimplification.

    What the intuition is missing is the notion of elastic scattering. In the intuitive description, the absorption and later re-emission of a photon are well-described as distinct events at particular points in time. The timing of the emission event is inherently random, so information about the timing of the absorbed photon is lost. This kind of scattering is called inelastic, and it disrupts the quantum coherence of the original light. Inelastic scattering tends to dominate when the energy of each photon is close to the excitation energy of the atoms; in the lingo, we say that the photons are nearly resonant with the atomic transitions. On the other hand, if the light is not nearly resonant, then a true absorption "event" is relatively rare. Instead, the interaction of the light with the atoms causes the light to distort without losing coherence. Mathematically, the distorted light can be expressed as some superposition of the original beam with the scattering patterns of the atoms, weighted according to how strong the light/atom interaction is. Less formally, the distorted light looks quite a bit like the input light, but usually slowed down and often with some bending.

    Once you have this extra detail in your head, some of the details of various substances become easier to explain.

    Why is glass so extremely clear? Since the molecules of (high quality) glass are arranged in a very regular lattice, their scattering patterns are very precisely lined up. As a result, any incoming light is distorted in a very clean, regular way; if you do the math, you find that the light slows down a fair bit but does not degrade at all in the ideal limit. Of course, the pattern will not be clean anywhere the lattice is disrupted, which is why the surface of a piece of glass and/or any cracks within it are much more visible than the bulk glass.

    How can water still be clear, but deform images seen through it? Unlike glass, water molecules are not locked into a lattice, so their scattering patterns are not well-aligned over long distance scales and can vary as the water sloshes around. Look at the bottom of a swimming pool on a sunny day, and you can see the interference peaks of the distorted light; they dance around as the water moves and the microscopic distribution of molecules changes.

    How does a metal mirror reflect light in a way that preserves the original image? Because of the details of metallic bonding, a piece of metal has a lot of electrons that can flow continuously from one atom to the next, allowing a wide range of smooth responses that are elastic in a way that true excitations are not. As a result, light/metal interactions tend to be elastic over a wide range of frequencies, and we see a preservation of light coherence that is simply not possible in substances whose electrons are more tightly bound to individual atoms.

    Think about it some more, and I'm sure you can come up with several other examples!
     
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  10. iceaura Valued Senior Member

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    That contradicts every source I have ever encountered that describes the molecular arrangements of glass. The large scale irregularity of the molecular arrangements in a glass appears to be a defining property of glasses - the key difference between glasses and crystalline solids such as diamond or quartz.
    https://en.wikipedia.org/wiki/Structure_of_liquids_and_glasses

    What I see is the macroscopic irregularity of the surface moving focal points and refractions created by the bulk material - you can get similar effects by moving a piece of transparent glass with an irregularly varying thickness, i.e. "wavy" glass.
     
    Last edited: Oct 17, 2016
  11. Fednis48 Registered Senior Member

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    Right on both counts. That gives me pause, actually, because I know that crystalline regularity can definitely enhance forward scattering of light. Since glass doesn't have that kind of regularity, why aren't the fluctuations visible? Is it just that atoms are so small their precise structure doesn't matter so much, and only the large-scale density affects things visibly?
     
  12. Q-reeus Valued Senior Member

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    Short memory spans. See http://www.sciforums.com/threads/all-photons-move-at-300-000km-s-but-dont.149931/
    Try posts #7, #12 and later. I prefer the polariton pov. That same link in #12 surfaced again in at least one later thread but can't recall where. Deja vu.
     
  13. exchemist Valued Senior Member

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    Yes, see Q-reeus's post. A number of us, myself included, had a serious go at that in the thread he has posted a link to. I recommend it.
     
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  14. Confused2 Registered Senior Member

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    Imagine embedding a double slit experiment (DSE) entirely in glass. Here defining 'work' as producing the well known interference pattern. Will the DSE 'work'? Will it work for single photons? If the answer is yes to both then can we hang on to the idea of 'a photon' interacting with 'a electron'?
     
  15. exchemist Valued Senior Member

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    Thanks for an interesting question. My first reaction is that I do not see why the experiment would not work. I do not see why the wavefunction for a single photon would not continue to explore both paths. It is interesting I suppose to envisage what the coupling of a single photon to the electrons in the medium might look like, but I don't immediately see why it would pose a problem. Do you think it would?
     
  16. Confused2 Registered Senior Member

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    Sorry - I think the point was covered by Q-reeus's link in an old thread (last few minutes).
    All Photons Move at 300,000km/s.... But Don't?
    In answer to exchemist - yes I think it would work. I defer to the video in Q-reeus's link.
     
    Last edited: Oct 17, 2016
  17. river Valued Senior Member

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    Hmm...

    What though of a flawles diamond .
     

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