= Anomalous dispersion
<Anomalous dispersion> is an interval in which the real <refractive index> decreases with increasing <frequency>, opposite to <normal dispersion>. It commonly occurs near a material <resonance>, where <optical absorption> also changes rapidly.
The <frequency> dependence of the complex <refractive index> determines both propagation <phase> and attenuation. A rapidly changing real part can give unusual <group velocity>, but that value alone does not specify the speed of a new signal in an absorbing medium. Measuring transmitted <intensity> together with <phase> or deviation across the band is therefore more informative than fitting a real <refractive index> while ignoring losses.
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