Could someone please explain how the speed of light, a supposed constant, can be slowed down in a medium? I checked Wikipedia and all they had to offer was this "In passing through materials, the observed speed of light differs from c. When light enters materials its energy is absorbed. In the case of transparent materials (dielectrics) this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore when the speed of light in a medium is said to be less than c, this should be read as the speed of energy propagation at the macroscopic level. At the microscopic level electromagnetic waves always travel at c." I guess all I need is a clearer explanation. Thanks.