On the concept of reaction field for electromagnetic fields in dispersive media

The concept of "reoction field" is introdoced for electromagnetic fields in dispersive media as a measure of the amount of the reactions of a radiation source on spatially distributed field detectors. It is found that the equations of the "reaction field" in the medium take the adjoint forms to those of the electromagnetic field. A reciprocity theorem having a clear-cut physical meaning is derived with the use of the concept of the reaction field. Some variational expressions are derived with the use of the reaction field.