Take light in glass. What could slow it down. I know that stuff is asorbed and then reemmited, but what about imperfections in the glass its self?? Say you have a piece glass that has impurities(little defects), or the desity of glass(not optical), the actual wave length of light, or for giggles the how bright the light is??? Would any of these affect the speed of light or would light simply travel around them???