In this paper we shall study the propagation of electromagnetic waves through media at rest in either an inertial frame or a rotating disk. We shall consider separately the problem of an inertial frame or a rigid rotating frame. This problem is associated with the SA6NAr [1] effect. The experiment of SAGNAr with a rotating interferometer is an example of an optical sensor to detect absolute rotation by measuring the fringe shift between two opposite traveling electromagnetic waves resulting from the rotation of the system. In recent years interest in the Sagnac effect has been renewed on account of the development of a self-oscillating version of the Sagnac interferometer. Experiments by MAeEK & DAVIS [2] and CHEO & HEER [3] used an optical ring laser attached to a rotating disk with a refractive medium in the path of two opposite moving light beams. They have shown that the rotation of the disk can be detected by measuring the frequency difference between the two beams. The original Sagnac experiment is shown in Fig. 1. The two opposite light beams result from the original monochromatic plane transverse wave of the light