Wow, that is not a small question nor does it have a simple answer! There are several textbooks with names like “interaction of matter with light” that would probably serve you better than any answer I could give here. (Unless, perhaps, you have a more specific question?)
for example:
https://www.amazon.com/Light-Matter-Interaction-Physics-Engineering-Nanoscale/dp/0198567669
The absorbance of a material depends on and is affected by its atomic structure. The value of the absorption coefficient of the material and thus its band gap depends on its absorbance value (you can use mathematical equations). Transmittance also behaves oppositely to absorbance, meaning that it also depends on its value. Therefore, all of these parameters mentioned depend on each other.
A band gap means transparency to em
Radiation up to a certain energy, the
absorption edge.
This is per lack of electron states or levels
in the gap.