Glass is an amorphous polymorph of silicon dioxide, melted and quenched so that grain boundaries grow uniformly and are small with respect to visible light. Its an oxide not a metal and has a large bandgap so visible light doesnt excite the electronic structure.

Now i can imagine how some gases and solvents are mostly transparent dielectrics, but im having trouble understanding why glass transmits light coherently.

How do the little crystals in glass manage to all polarize the same way if they are arranged randomly and aren't free to move? I read glass molecules do move like a magma but very slowly. Is it just that the negative charge polarizes like in a leyden jar?

If thats the case why doesnt the motion of a gas or a fluid affect the coherent transmission of light?

  • $\begingroup$ @Chair I am not sure the question is a duplicate. The OP should clarify how "coherently" should be intended in the context of the question. $\endgroup$ – GiorgioP Feb 25 '19 at 7:05
  • $\begingroup$ There are no ‘grain boundaries’ in an amorphous solid. $\endgroup$ – Jon Custer Feb 28 '19 at 4:19

You are asking why glass is transparent. On the quantum level, there is a debate on this site what really happens. I will give you the explanation that I find to be logically right.

The answer to your question is on the quantum level elastic scattering. This is the only way the photons:

  1. keep their energy level

  2. keep their phase

  3. keep their relative energy level

  4. keep their relative angle

If you look into a glass window, you will see two things:

  1. a coherent mirror image of what is on the other side of the window, this is what you are asking about, the glass seems transparent

  2. a specular reflection, that is a mirror image of yourself. In this case the glass is acting as a mirror.

Now how can a glass window do both at the same time? Be transparent, and be a mirror too?

The solution is elastic scattering. In the case of a glass window acting as a mirror, this is called a specular reflection. Specular reflection (mirror image) is elastic scattering.

Now what is the difference between when the image goes through the glass, and when it is reflected? Only the angle. They are both elastic scattering. In the case of the glass being seethrough, photons get elastically scattered through the glass. In the case of the glass acting as a mirror, the photons get elastically scattered off the mirror's surface.

Now you are asking why the motion of gases does not affect the mirror image. In the case of air, this is called an elastic scattering too. But the real reason why this does not matter and the mirror image is kept, is because the speed of the gas molecules' motion that you are asking about, is almost zero compared to the speed of photons. Remember photons always travel at the speed of light in vacuum (and they are traveling in vacuum inbetween the molecules) compared to any object that has rest mass. Now all the molecules do have rest mass, so the photons are traveling at the speed of light compared to them (molecules). Since the molecules' speed is negligible compared to the speed of the photon's, the motion of the gas molecules do not affect the mirror image. Photons fly through the gas as if the gas molecules were not even moving at all.

  • $\begingroup$ But why they keep the relative angle? $\endgroup$ – Alchimista Feb 25 '19 at 9:17
  • $\begingroup$ Good question. The lattice decides the change of angle. The lattice structure is almost the same everywhere in the glass. That is why the relative angle of the photons is kept. You can see this when there is a density change in the glass, or a broken part, there, the lattice structure changes, and the angle of the photons changes too. $\endgroup$ – Árpád Szendrei Feb 25 '19 at 15:24
  • $\begingroup$ Well yes is down to get a homogeneous refraction index. Is the microscopic view based on scattering that confuses me. $\endgroup$ – Alchimista Feb 25 '19 at 15:39
  • $\begingroup$ Yes it is very hard to understand. First you have to understand the double slit experiment, and that the photon gets split into partial waves and those partial waves interfere with each other and so a single photon can interfere with itself. This is the same thing what happens with photons in the lattice. The molecules are the slits and the photon gets split into partial waves as it travels through the lattice and interferes with itself. That is how it changes angle when it enters a different medium or when the medium changes. $\endgroup$ – Árpád Szendrei Feb 25 '19 at 15:54
  • $\begingroup$ Yes also clear. Not all direction are allowed $\endgroup$ – Alchimista Feb 25 '19 at 15:59

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