Real Fringes in the Sagnac and Michelson Interferometers

In describing the Michelson interferometer, it is usually stated that in order to observe fringes an extended source must be used. It has to be realized, however, that this requirement applies only to the formation of virtual fringes. Actually two distinct sets of fringes exist; virtual and real. Rarely, if ever, is attention brought to the existence of the latter type. The existence of the real fringes was first noted by the authors using the Sagnac interferometer, which is described in this paper. This little-known interferometer and the Michelson interferometer are both amplitude-splitting types. Real fringes were obtained for both a point source and an extended source while virtual fringes were, of course, only visible for an extended source. Similar sets of real fringes were also observed for the Michelson interferometer using either an extended or a point source. An analysis of the formulation of real fringes is presented.