Photoelastic modulators (PEMs) consisting of a plane-parallel dielectric plate exhibit multiple internal reflections like those in a Fabry- Perot interferometer. If a PEM is illuminated by a coherent radiation source at normal incidence, a modulation of the intensity of transmitted light can be observed. In a recent paper by Oakberg, the modulated interference effect was interpreted as periodic thickness variations of the PEM plate. We discuss three different models that describe interference modulation in PEMs. Then, it is experimentally demonstrated for a PEM with a symmetric optical element that the periodic variation of the refractive index in the PEM plate is responsible for this modulated interference effect.