Time-dependent perturbation theory. Interaction of electromagnetic (EM) radiation and atomic systems. The Kramers-Kronig relations. EM fields and their quantization. Spontaneous and induced transitions. Homogeneous and inhomogeneous broadening. Absorption and gain. Optical resonators. Laser oscillation. Transverse modes and Gauss beams. EM field in anisotropic crystals. Modulation of EM waves.