The technical name was kinda given in the question, and I'm guessing that's what they want: Dispersion. It refers to the phenomenon of different frequencies traveling at different speeds through a medium. This is due to the fact that the relative electric permittivity is a complex quantity that is a function of frequency. The relative electric permittivity, [tex] \epsilon_{r} [/tex], is related to the speed of the wave in the medium by the relation:
[tex]c' = \frac{c}{ \sqrt{ \epsilon_{r} } } [/tex]
Since this speed is a function of permittivity, and permittivity is a function of frequency, speed is a function of frequency.
Probably more info than you needed....
