What happens when light crosses the interface between two transmitting materials with different refractive indeces? Does anybody know what its interaction is between the two different media and why does this interaction cause it to change velocity? That light changes velocity indicates there must be some interaction. However light passing through a medium of higher refractive index is not changed by the interaction, because when it reenters the medium of lower refractive index it resumes its original properties. Does anybody have a non mathematical explaination for this?