A simple method for the measurement of the refractive indices of parallel plate samples is described that uses the shift of the interference pattern when rotating the sample in one arm of a Mach–Zehnder interferometer. The theoretical fringe pattern shift is calculated and the accuracy of the method is derived. A typical accuracy of about 10−4can be achieved at low refractive indices. The method is especially useful when—because of other reasons—the samples are provided as plates or when prisms are difficult to fabricate.