> If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question. U

> If it were true we would see sharp changes in the observed speed of light at wavelengths depending on the absorption spectrum of the material in question. Uh, we do. The imaginary portion of the index of refraction near a resonance generally looks roughly like a gaussian, and the real portion does have jumps that look a bit like tanh.