anomalous dispersion

Anomalous dispersion in physics refers to a reversal of the normal order of separation of components in certain refraction spectra. This occurs when the frequency of radiation is close to the natural electron frequency.12

In normal dispersion, the refractive index of a medium increases with frequency, causing violet light to bend more than red light in a prism. However, in anomalous dispersion, the refractive index decreases with increasing frequency. This can lead to a refractive index less than unity, meaning the speed of light in the medium at that frequency is greater than its speed in a vacuum. It's important to note that this refers to phase velocity, and the propagation velocity of actual signals (group velocity) remains less than the speed of light, thus not violating relativity.2