Optical dispersion is the dependence of refractive index or phase velocity on frequency. It gives an optical prism wavelength-dependent deviation and causes chromatic aberration.
In normal dispersion, the refractive index increases with frequency, , equivalently for vacuum wavelength . Transparent regions away from isolated absorption resonances often have this behaviour. Near a resonance the slope can reverse, producing anomalous dispersion accompanied by absorption.
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.
The Abbe number compares refractive strength with visible optical dispersion. The traditional reference lines are near nm, respectively. A large means weak relative dispersion.
An Abbe diagram plots refractive index against Abbe number to show available optical glass families. Many diagrams put increasing to the left. Ordinary crown glasses have larger than ordinary flint glasses, while modern compositions occupy a broader region.
Articles by others on the same topic
Dispersion in optics refers to the phenomenon in which the phase velocity of a wave depends on its frequency. This effect is most commonly observed when white light passes through a prism or other transparent medium, resulting in the separation of light into its constituent colors, typically represented as a spectrum ranging from red to violet. The underlying reason for dispersion is that different wavelengths of light travel at different speeds in a medium.