The negentropy principle of information (Brillouin's principle) is formulated in a positive form in order to express the amount of information received by a physical system in terms of physical entropy. This principle is applied to the problem of information transmission by the electromagnetic field and information characteristics of both wide-band and narrow-band channels (capacity and minimum energy per unit of information) are calculated on the base of statistical thermodynamics. The usual information-theoretical derivation is also given. Exact quantum-mechanical expressions, which give the well-known Shannon's formulas in the classical limit case, are obtained. On the other hand, the effect of the quantum nature of electromagnetic field is revealed, the paradox of infinite capacity in the absence of noise being eliminated. Problems concerning the number of degrees of freedom, polarization states, and nonequilibrium noise are also discussed.