Introduction to Electrodynamics/Chapter 8/4/1

8.4.1 The Frequency Dependence of \epsilon, \mu, and \sigma

If everything were as we said it was, so far, then prisms wouldn't work and the sky would not be blue.

The media we shove EM waves through has different values of \epsilon, \mu, and \sigma according to the frequency we are shoving through them. This changes the speed of light in those media depending on the frequency. This phenomena is called "dispersion". Media which exhibit dispersion are called "dispersive".