The speed of light in a vacuum is constant and the apparent slowing down of light as it traverses a given medium is because of the time it takes for the light to interact (be absorbed and re-emitted) with the particles that makes up the medium.
If the above is correct, here are my questions:
Is the light travelling through a medium interacting exlusively with the medium’s electrons? Or does light also interact, say, with its protons?
When we say that the speed of light in a vacuum is constant, are we referring to a classical vacuum or a quantum vacuum?
Assuming that light does interact with the virtual components that comprise the quantum vacuum, are all quantum vacuums the same in this regard? If not, does the speed of light vary (in a measurable way) depending on the quantum vacuum at hand?
Thank you (and, as usual, I apologize if these questions are better considered as proof of my fundamental lack of understanding of the subject).