Hi I would like to ask that what makes a transparent medium transparent ? What happens when a photons of light passes through our atmosphere ? I suppose that visible spectrum of light passes through atmosphere without causing excitation of electrons of gases' molecules, but what happens when photon doesn't excite electron and still encounter matter does it just passes through it or bounce back ? In other words what is refraction? Is it passages of photons through interstitial spaces or absorption and then re-emission of same photons?