A medium with electromagnetic properties modifies an electromagnetic field imposed on it. The response of some media may be described satisfactorily in terms of induced dipole moments, and this is particularly the case for the response to static fields. The response to a fluctuating field may also be described in terms of the induced current density. This alternative description is used widely in plasma physics and is emphasized in the approach adopted here.

In Part Two the nature of electromagnetic responses is discussed in Chapter 6 and general properties of response tensors are summarized in Chapter 7. An understanding of the material in these two chapters is important in the discussion of waves in media in Part Three. The remaining three Chapters in Part Two are more of the nature of reference material. Although much of the material in these Chapters is referred to and used in the remainder of the book, a detailed understanding of this material is not essential before proceeding to Parts Three, Four and Five.