Partial dispersion

The partial dispersion of optical glass and other optical materials describes the strength of the dispersion of the material between two specific wavelengths ( light colors ). It is therefore a measure of the difference in the refractive index at these two wavelengths.

For optical materials it is important to know their refractive index and its dependence on the wavelength ( dispersion ). The Abbe number is the most important measure of this dependence. It indicates whether the refractive index generally changes strongly or weakly with the wavelength.

In order to describe the dispersion more precisely, one defines partial dispersions for different wavelengths x and y . They are usually given as relative partial dispersions : ${\displaystyle P_{xy}}$

${\displaystyle P_{xy}={\frac {n_{x}-n_{y}}{n_{F}-n_{C}}}}$

Here, the refractive index at the Fraunhofer lines F (wavelength 486.13  nm ), and accordingly (656.28 nm) in the Fraunhofer lines C. ${\displaystyle n_{F}}$ ${\displaystyle n_{C}}$

The partial dispersions in different areas of the wavelength scale are important if you want to correct the secondary spectrum in optical systems such as objectives ; see apochromat . For this you need materials with an unusual course of the refractive index, e.g. B. long crown glass, which has a large partial dispersion for blue light, or short flint glass with a particularly small partial dispersion for blue light.

See also

• Sellmeier equation