  This paper is devoted to the theory of quantum electromagnetic field in an
optically dense medium. Self-consistent equations describing interaction
between a quantum field and a quantum dielectric medium are obtained from the
first principles, i.e., outside a phenomenological description. Using these
equations, we found a transformation (of the Bogoliubov transformation type)
that converts the operators of the "vacuum" field into operators of collective
perturbations of the field and an ensemble of atoms, that is, photons in the
medium. Transformation parameter is the refractive index of the wave mode
considered. It is shown that besides the energy of the collective
electromagnetic field, the energy of photons in the medium includes the energy
of the internal degrees of freedom of the substance and the energy of
near-field dipole interaction between atoms in the polarized medium. The
concept of negative energy photons is introduced on the basis of
self-consistent equations.
