In some dielectric media, the dielectric constant, , is complex. According to Equation (772), this leads to a complex wavevector, (assuming that the angular frequency is real). Thus, for a wave propagating in the -direction, we obtain
Up to now, we have tacitly assumed that is the same for waves of all frequencies. In practice, varies (in some cases, strongly) with the wave frequency. Consequently, waves of different frequencies propagate through a dielectric medium at different phase velocities, leading to the dispersion of wave pulses. Moreover, there may exist frequency bands in which the waves are attenuated (i.e., absorbed). All of this makes the problem of determining the behavior of a wave pulse as it propagates through a dielectric medium a far from straightforward task. Of course, the solution to this problem for a wave pulse traveling through a vacuum is fairly trivial: that is, the pulse propagates at the velocity without changing shape. What is the equivalent result for the case of a dielectric medium? This is a significant question, because most of our information regarding the universe is obtained from the study of electromagnetic waves emitted by distant objects. All of these waves have to propagate through dispersive media (e.g., the interstellar medium, the ionosphere, the atmosphere) before reaching us. It is, therefore, vitally important that we understand which aspects of these wave signals are predominantly determined by the wave sources, and which are strongly modified by the dispersive media through which the signals have propagated in order to reach us.