Well, calling it transparency only gives a name to “see-through-ability”. So let’s look at the physics. If a photon enters a material (solid, liquid, or gas), basically three things can happen:
- The photon can be scattered by the material,
- The photon can be absorbed, or
- If the photon is neither scattered nor absorbed, it will be transmitted - that is, the material will appear transparent.
So the question of whether or not a material will be transparent boils down to whether it can scatter or absorb photons. Now, there a variety of mechanisms by which either of these (scattering or absorption) can occur, but let’s look at just a few:
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If there are free “conduction” electrons, these electrons can easily absorb photons and which are excited to a higher energy state in the process. When the electrons relax to their original energy, a photon is re-emitted. The absorption of the original photon is very strong, so it can’t penetrate very far. So this material is not transparent, but the fact that photons are re-emitted makes it reflective. This is the behavior of metals, and in fact many of the characteristic properties of metals - reflectivity, electrical conductivity, and thermal conductivity - are due to the conduction electrons.
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If there are no conduction electrons, then a photon can still be absorbed if a valence electron can be excited to a different (higher) energy band. In this case, the energy of the photon must be large enough to promote the electron across the “band gap” separating the two bands. Now, ordinary glass is electrically insulating (no conduction electrons) and has a band gap of about 9 eV. The energy of a visible light photon is only 2-3 eV, not enough for the photon to be absorbed, so it will simply be transmitted. This is why glass is transparent to visible light. It is also why quartz - which is chemically quite similar to ordinary glass but crystalline - is transparent (if there are no defects - see below).
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Even a material that cannot absorb photons by either of the mechanisms described above might still not be transparent, if there are defects in the structure that can scatter light. A common example is ice, which if carefully frozen can be nearly perfectly transparent but becomes translucent if there are lots of bubbles. Similarly, quartz can be translucent if it is polycrystalline (composed of many small crystals), in which case it is the boundaries between the crystals scatter the light.
Despite being long-winded, the above discussion is really oversimplified since it ignores things like the frequency (or energy) dependence of absorption and reflection, not to mention things like color centers. But the gist of it is that any material incapable of either scattering or absorbing photons will necessarily be transparent.
On to the issue of solids, liquids, and glasses. Everybody “knows” what a solid is, and what a liquid is. Glasses don’t fit neatly into either category, but the problem is not with the glasses, it’s with the categories. Like so many other things, once you start looking at hard cases, definitions that once seemed obvious become problematic. (We can blame our need to put things into distinct categories on Aristotle, I suppose, but that’s a different discussion.)
Traditionally, a liquid is defined as a material incapable of supporting shear stresses. (Shear stresses tend to distort a body, as opposed to hydrostatic stresses which change volume without distorting shape. Liquids can support hydrostatic stresses, which can be quite large at the bottom of the ocean, for example.) Of course, this is not a binary condition - materials can show a range of resistances to shear stresses, which we quantify through the viscosity. Liquids have a low viscosity, with some liquids (molasses) being more viscous than others (water). Solids, of course, have a very high (effectively infinite) viscosity. So the question becomes, where precisely do you draw the line - that is, what value of viscosity separates liquids from solids?
Even this question is not so simple as it seems, because it depends on how long you are willing to wait. For instance, I might put a shear stress on a material and observe how it deforms in response. If I’m impatient and walk away after a second, I might not observe any deformation at all. But if I wait hours or days, then deformation might become apparent. And, of course, whether or not I can observe any deformation depends on the tools I use to measure it (e.g. my eyes vs a laser interferometer). So, as a practical matter, we need to chose some more-or-less arbitrary value of viscosity as an operational definition of what separates a liquid from a solid.
Imagine we have molten silicon dioxide, which we then cool to room temperature. The viscosity of the liquid will increase as the temperature drops. At some point, the viscosity will increase to the level we have defined as separating solids from liquids; we say that the liquid has gone through the “glass transition” and is now a solid. But of course the transition is not abrupt; it occurs over a range of temperature and the viscosity changes continuously over this range. (And, in fact the glass transition temperature is not an inherent property of the material but depends on things like the rate at which the material is cooled). Again, this is oversimplified, because other properties of the material (such as the heat capacity) also change at the glass transition. But the basic point is that there is no clear-cut distinction to be made between the solid (glass) and liquid states of the material.
The question of whether old glass (e.g. in cathedral windows) flows has already been addressed in this thread - it doesn’t, at least not measurably. Another way to see this is to consider the glass vessels made by ancient Egyptians; if cathedral windows flowed noticeably in hundreds of years, then glass vessels made thousands of years ago should have flowed into puddles - but they haven’t. The viscosity of glass at room temperature is so high as to be effectively infinite.
If, despite this, you still want to think of glass as a liquid, consider this: Lead (the metal) is clearly a solid, yet will flow (creep) at room temperature. This can be a problem if lead is used as a roofing material.
The question of structure is largely irrelevant here, except to confuse people. Yes, the atomic-scale structure of a glass is more similar to that of a liquid than to a crystalline solid, but that has little to do with whether a glass is a solid or not. In fact, lead (which can flow at room temperature) is crystalline, while window glass (which does not flow at room temperature) is amorphous.
Sorry for the long-winded post, but I hope it serves to clarify some of these issues.
