I am trying to reconcile the optics explanations (from “Why glass is transparent?”) with the bits I learnt about band gaps and fermi levels. It is hard to write down a single question for now, will have to edit this question to suite the StackExchange format later. I welcome all forms of comments and answers.
My current understanding is that, glass being transparent (to visible light) is because it is not absorbing any photons at all. Its structure is so rigid ( meaning the energy band will be narrow and far apart ) the visible light just didn’t happen to contain the right amount of energy to make any electrons in any fully filled band to jump to a partially filled band. If there is no jumping then there is no absorption. [NOTE, This understanding is proven to be correct and confirmed]
I also think, the propagation delay in glasses is caused by impurities and crystalline imperfections so that some absorption occur.
I can not explain why a pure silicon crystal is not transparent. Visible light contains just the right amount energy to make electrons in silicon crystal to jump band? It is a semi conductor so it is probably less draconian about its electrons freedom. [NOTE, this question is solved, as the visible light's energy is 1.65~3.1ev, and minimum required energy for silicon electron jump is 1.11ev, which is its band gap energy]
Visible light is a very very narrow band, I would expect many solids in this world to be transparent, I would expect lots of transparent insulators. But that's not the case in real world. There are way more insulators than transparent objects.
I do not know how “elastic” absorption works. Inter-molecular bond or inner-molecular bond, shouldn’t all of the electrons in whatever bonds be in the same fermi sea and be subjected to the same rule?
I am also aware of the uncertainty principle at this scale. light can behave strangely just by itself. I learnt how to calculated diffraction but never learnt the true cause. I am not sure if that has anything to do with this topic.