A while back, I asked a question about the absorption of different wavelengths by matter (see this post). I received some excellent answers that prompted to undertake some more in-depth research, especially concerning band structures / energy bands in materials. I did this and have a better understanding of this topic now, though it is still very rudimentary.
I will try to explain how I currently understand this topic, and then formulate my questions. By my knowledge, when multiple atoms, each with only a few discrete energy levels come together to form a more complex object, their energy levels overlap. However, due to the Pauli exclusion principle, no two electrons can exist on the same exact energy level, therefore each energy level breaks apart into 2 new levels. Expanding this process onto a large number of atoms results in complex materials having continuous energy "bands" instead of just a few discrete energy levels.
My first question concerns the absorption of light by electrons in these energy bands. Can a photon only be absorbed if it will transition an electron into a higher band? Or can it absorb a photon and just transition to a higher energy level within its current band? The first option would limit the amount of wavelengths that could be absorbed, while my second assumption means that many more wavelengths could be absorbed.
Additionally, my understanding is such that the size of a materials bandgap also determiens its conductivity, where large band gap (e.g glass) = insulator, moderate bandgap = semiconductor and no bandgap = conductor. I also read frequently that the band gap is responsible for determining the transmittance of the object. For example, a large bandgap of an insulator would mean that visible light could not excite photons across the band gap, making the object transparent to visible light. However, if this is the case, how can it be that some insulators like glass are transparent to visible light, but others such as wood are opaque?
I'd be happy if anyone could clear these misunderstandings up, or even recommend a good source. Thank you!