How does a photon "know" to pass through without interacting? The usual explanation given for transparency is that when the energy of a photon is smaller than the band gap energy of an atom, the photons don't interact with the electrons and pass through, so the material is transparent. But what stops the photon from interacting with the electron? How does it "know" that the band gap energy is too large? Is there a model of this 'non interaction', or 'passing through' somehow - perhaps as some sort of a dynamical process, using a quantum mechanical model of the electron and the changing potential caused by the passing photon's EM wave? Or is QED needed to understand this? Whatever the theory, what is a qualitative way to describe the absence of the interaction?