text: The speed of light is dependent on the medium in which it is traveling.  Light CAN be slowed down.\n\nIn a vacuum, light travels at exactly 299,792,458 m/s.  However, in other medius, like air or glass for example, light will slow down.  The more optically dense the medium , the more it slows down.  We can define a quanity, called the index of refraction (n), which will help us determine the speed of light in a substance.\nThe higher the index of refraction, the slower light travels.\n\nThe velocity at which light propigates (v) is inversely proportional to the index of refraction (n).\nv = c/n\nwhere c is the speed of light in a vacuum.\n\nIn a perfect vacuum, the index of refraction is exactly 1.  Air has an index of refraction of just slightly above 1, but often, since it is so close, we can just round it off to be 1.  Glass however (depending on the type) can have much higher indexies of refraction, (~1.3 - 1.5 roughly).\nWhen air moves from lower index of refraction to a higher index of refraction (or vice-versa) it is bent.  This bending of light is due to the fact that the light is slowing down as it enters the medium, which is the case in a prism.\nDifferent wavelengths of light have different indexies of refraction, all wavelengths do not slow down equally.  This is the reason why blue light is bent more easily than red light and appear at opposite ends of the spectrum.\n\nLigh DOES slow down as it enters a more optically dense medium.