Hey! I just saw a video on youtube from the guys at nothingham university (sixty symbols) where they explained that some materials are transparent due to the energy gap between the excited states of the electrons in the material. If photons come in at lower energies they will not interact with the material and pass right trough. The video is found here: However, if the photons pass right trough, how come the speed of light slows down in for example glass? I've heard that the reason that this happens is precisely because the photons get absorbed and reemitted. Both quantum and classical explenations are appreciated.