In the frame of ray optics it is shown how to calculate the permittivity tensor of a non-uniform medium. This tensor describes correctly the resonances between particles and electromagnetic waves. The resonances in the inhomogeneous media take into account not only the radiations existing in the uniform media (Cherenkov and cyclotron radiations) but also the bremsstrahlung and transition radiation which are due to the media inhomogeneity. We consider the collective curvature radiation from the relativistic plasma placed in the strong curvilinear magnetic field.