Problem for finding the criteria of belonging the vector fields to the set of electromagnetic fields is formulated. The existence and uniqueness of the solution of this problem are investigated. The desired criteria for the set of alternating electromagnetic fields in the frequency domain in the case of isotropic media are formulated. It is shown that these criteria are shared set of alternating electromagnetic fields in linear isotropic media into two subclasses. Are examples of the fields belonging to these subclasses.
