The diagram in Fig. 22.1 provides a schematic summary of this textbook and serves to classify the place of the microphysical theories of radiative transfer and WL within the broader context of Maxwell's electromagnetics. Although we have been using the adjective “microphysical” in order to emphasize the back-traceability of both theories to the MMEs, it can also be said that these theories have a mesoscopic origin. Indeed, the term “mesoscopic physics” refers to a size regime that is intermediate between the microscopic and macroscopic and is characteristic of a region where a large number of particles can interact in a correlated fashion. The direct computer solutions of the Maxwell equations described in Chapter 18 demonstrate indeed how the “macroscopic” regime of radiative transfer and WL emerges from the “microscopic” particle-level regime of Maxwell's electromagnetics upon averaging over random realizations of a multi-particle group. Extensive discussions of mesoscopic optical phenomena can be found in the monographs by Sheng (2006) and Akkermans and Montambaux (2007).

Besides being a one-page summary of the book, Fig. 22.1 also helps identify problems that still await solution. First of all, by using the frequency-domain MMEs as the point of departure, we have completely excluded from consideration such phenomena as emission of electromagnetic waves and frequency redistribution, as well as situations involving pulsed illumination.