When radiation enters most transparent media, the ultraviolet photons and infrared photons are absorbed while the frequencies of visible light are transmitted. As the photons, the quanta of electromagnetic waves, move through the media, the electrons in the medium begin to move [vibrate] in response to the electruc field of the traveling radiation as it passes and exerts electric forces on the charged particles. The electrons, in turn, now start producing their own electric waves which superposition with both those of other electrons as well as with the original electromagnetic wave of the traveling photon. Since the waves produced by the vibrating electrons are moving much slower that that of the photon, the net result is a final wave moving through the medium at a speed slower than c.

In the following video Fermilab's Dr. Don Lincoln of FermiLAB explains this "slowing down of light" as it moves through a transparaent medium. He also explains why the "scattering" and "absorption" explanations for this phenomena are incorrect along with why light "apparently" speeds up when it exits the medium.

As Allen Everhear stated in an online forum: "The constant c is the speed of light in vacuum, far away from strong gravitation. In a transparent media the oscillating electromagnetic fields of light wiggle the electrons of the media. Wiggling electrons produces a light wave slightly out of phase with the incident wave. The combined incident and induced waves move through the medium slower than light in vacuum."

This question of why light slows while traveling through a medium is extremely complicated and still remains a subject of debate among physicists. Quantum mechanical events are prevasive in the more detailed arguments. Let this page and its links spark your curiosity and stimluate your further study.