I'm wondering about small changes of phase due to a refractive medium. For example, suppose there is an emitter of radio or light waves, and two detectors equidistant from the emitter. One of the detectors is behind a medium, while the other is a straight shot through a vacuum. How would the difference in the detectors evolve over time? Would it just be a constant phase shift, or would the angular frequency of the refracted wave change? How can I think about this properly? Thanks in advance.