I read the HyperPhysics entry, but all I seemed to find was similar to what you guys said. I think most transparent materials are dielectrics, but I have heard of transparent conductors, so I don't think that could be it, either.
HELP! I'm drowning in my confusion.
Dave
Light can either be stopped by reflecting it, which occurs at any interface, or by absorbing it. Most white powders (clouds, sugar, snow) have a lot of interfaces with different indexes of refraction, so reflections occur and very little light can directly pass through. This is dielectric reflection, which is always only partial reflection, except at angles where total internal reflection occurs.
Classically, conductors will move in response to the electric field of light waves, perfectly stopping the light and reflecting it backwards. This is why metals can be polished to a "mirror finish".
To absorb light, the material needs to be able to be able to make a quantum jump from one state to another. For an individual atom, these occur at specific energies and hence specific frequencies of light, which means only certain colours can be absorbed. In a solid, however, these get spread out so a range of energies can be absorbed. However, there are only
bands of these: an electron needs to either get enough energy to jump over the difference between a band, or it needs to get a small enough amount of energy to stay inside a band. Bands can overlap as well, but for insulators and semiconductors the important ones are separated.
In an insulator, the electrons fill up the lowest energy bands, so they need to get enough energy to jump over a band. For something like diamond, the energy required (called the band gap) is about 5 electron volts, which is roughly 5 times the energy in a photon of visible light*. As a result photons can't be absorbed, and diamond crystals are transparent.
To get a transparent conductor, you first need to make sure no absorption happens. Then you need to have a conductor, but not one that responds to the electric field of visible light. To do this, we need to think about what happens to the electrons that are doing this conducting: the electric field is shoving them one way, and then the other, and they are moving in response. But the electrons are also attracted to the ions they came from.
The bulk electrons are attracted to the bulk ions like they were connected by a spring: if you shift the electrons, they bounce back and forth, at the natural frequency. For conduction electrons, this is the
plasma frequency. Electrons can't easily move faster than the plasma frequency, and as a result the material stops acting like a conductor: it can't respond to the electric field of the light fast enough, so it can't cancel the field inside the conductor, at least not entirely. So if you just adjust the plasma frequency (which can be done in semiconductors by adding fewer conduction electrons) to below the frequency of visible light, and use a material with a band gap large enough to let visible photons through, the material will conduct and be transparent at the same time.
This is rather unusual though, most conductors are reflective, and possibly coloured if they also absorb certain energies. It's also a good idea to keep it thin, as some of the light will still be reflected.
Of course, you can describe conduction quantum mechanically rather than classically, but it doesn't change the results all that much. And if you have a good imagination, you can see how this stuff would apply to glass and liquids (disordered and dense) and gases (disordered and not dense). The article in the OP is especially bad about that, as it mentions glass and candy as examples of how disorder leads to clearness. Crystallized glass is largely quartz, and crystallized sugar is rock candy, both of which are transparent.
*Note that while you could hypothetically use 5 photons at once and have enough energy, this is really unlikely. If it can't be done with 1 photon, it probably isn't a visible effect.
