## EM waves in a dielectric

In general the electronic susceptibility, χ, is a function of frequency and is complex for a dielectric medium.
So what are the implications of this for the propagation of EM signals in a dielectric medium?

Since the electronic susceptibility varies with refractive index, and the refractive index varies with wave number k, then the wavenumber is also complex - so it has an imaginary component. What does this mean in terms of wave propagation (phase shift with frequency)?

 Quote by ian2012 In general the electronic susceptibility, χ, is a function of frequency and is complex for a dielectric medium. So what are the implications of this for the propagation of EM signals in a dielectric medium? Since the electronic susceptibility varies with refractive index, and the refractive index varies with wave number k, then the wavenumber is also complex - so it has an imaginary component. What does this mean in terms of wave propagation (phase shift with frequency)?
It means the medium is dispersive. What that means is that planewaves of different frequencies travel at different speeds and have different phase shifts. In the time domain it means that there is distortion of a pulse as it travels. It may also mean that there is absorption of energy in the medium resulting in attentuation with distance.