It's a consequence of the relationship between absorption and index of refraction in glasses, which follow the Karmers-Kronig relationship [0]. As the waveleng

It's a consequence of the relationship between absorption and index of refraction in glasses, which follow the Karmers-Kronig relationship [0]. As the wavelength approaches atomic resonance, the index increases or decreases depending on which side of the absorption peak the wavelength is. 0. https://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relations https://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relatio...