The phase shift is dependent on both the resonance frequency of the medium and the incident light. In the case of X-rays, the refractive index can go below 1, r

The phase shift is dependent on both the resonance frequency of the medium and the incident light. In the case of X-rays, the refractive index can go below 1, resulting in total external reflection for a few tenths of a degree. Even in this case, the speed of the X-ray itself stays below that of C, even though the "wavelength" appears to be longer than in vacuum.