While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is re-emitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then re-emitted as an electromagnetic wave. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium.
Different materials cause a different amount of delay due to the absorbtion and re-emission process.
Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling. For more information on physical descriptions of waves, visit The Physics Classroom . Other animations can be seen at the Multimedia Physics Studios .