I found this question from a different angle to be very helpful when understanding transparent vs opaque. And colors. Other folks might appreciate this approach too.
“Why shouldn’t photons go on through solids?”
We think of solids as, well, solid. But really they are mostly space. 99.999(and a bunch more 9’s)% space. So why wouldn’t a photon just slide on through? Electron orbital states to the rescue. If a photon gets near an atom’s electron, it can make the electron jump states. Jump to a higher orbit. The photon is absorbed in the process. But only if there is a close enough match between the energy of the jump and the energy of the photon. The gap you spoke of.
If there isn’t a close match, the photon just keeps on goin’.
Every substance has different gaps. If they are gaps that match visible photons, those colors are absorbed. For diamond, none of the gaps match visible photons. For graphite, all of the gaps match. For rubies and blood, non-red colors are absorbed. For emeralds and leaves, non-green is absorbed. And so on.
