Classical (non-quantum) explanation for the transparency of glass? Is there a classical explanation for the transparency of glass? How did physicists explain the transparency of glass before quantum mechanics?
 A: As you point out, the nature of interaction between matter and light was first addressed by Niels Bohr, when he proposed the Bohr atom
Why does each element have a specific spectrum associated with it? Because for each element the energy levels of the electrons around the nucleus come out differently.
The development of quantum mechanics gave scientists the means to address the nature of transparency of substances.
The point is: when you don't have the means to address a question you may try, but without the means what you are doing isn't science.
Part of being a scientist has been and will always be that you choose your battles with care.
I happened to come across a striking historical example of that just a few days ago. Wikisource has a transcript of Maxwell's On physical lines of force
In the introduction Maxwell mentions that while a lot is known about electricity and magnetism, it is not known whether flow of current is a single flow (either positive or negative carriers of charge) or that it is always the case that both positive and negative carriers of charger participate in current flow.
In Maxwell's time the means were not available to address that issue, so Maxwell didn't.
Instead Maxwell directed his efforts to questions he could address with the means available.
As we know, Maxwell's unification of theories of Electricity, Magnetism and Light is one of the greatest achievements in the history of science.
Transparency
Before the development of quantum mechanics I don't think that physicists even considered addressing the question of transparency.
If you cannot formulate a theory then whatever ideas you offer on the subject is speculation. Speculation isn't in itself wrong, but when you are speculating you don't know whether you are making any progress.
