Glass refractive index vs wavelength, exceptions?

[lalbatros]

Normal glasses have a larger refractive index for blue than for red.
Are there special glasses that differ in this respect?
Or maybe are there some minerals that behave differently, still being transparent in the visible spectrum and colorless?

Thanks to tell me if you know something about that.

 

[Claude Bile]

There are two manifestations of anomalous dispersion (short wavelengths having lower n) in glass. The first manifestation is due to absorption peaks in the glass. Near the short wavelength side of an absorption peak, dispersion will be anomalous. Glasses can be doped to control at what wavelength ranges anomalous dispersion will occur. The second manifestation is due to the overall material dispersion in the glass. Glasses possesses a characteristic wavelength where the dispersion in the glass is zero. Above this characteristic wavelength, normal dispersion (short wavelengths having high n) prevails, below this wavelength, anomalous dispersion is predominant. In silica for example, the zero-dispersion wavelength is 1.55 microns - which is why telecommunications networks primarily use this wavelength range for signal transmission in optic fibres.

Claude.

 

[lalbatros]

Thanks a lot Claude.
Very instructive answer.