Dispersion Abbe Number

The refractive index measurement at different wavelengths also referred to as dispersion, helps to characterize transparent optical materials. Anton Paar’s Abbemat MW can be equipped with up to eight wavelengths.

The refractive index depends on the wavelength of the light. This wavelength dependency is also referred to as dispersion. Dispersion can, for example, be used to spread white light into its color components with a prism. The different colors are light waves with different wavelengths, which together appear as white light. Due to dispersion, there are different refractive indices and thus different refraction angles in the prism, spreading the white light into a rainbow.

In the same way, dispersion is also the cause of the “chromatic aberration” seen in photography. It is an important property for characterizing all kinds of optical components.

The velocity of light in a material depends on its wavelength. The smaller the wavelength, the lower the light’s velocity. Therefore, blue light of short wavelength will be more refracted than red light of longer wavelength. This means that every material has different refractive indices for different wavelengths.

Due to the different refractive indices, white light that is refracted, for example, at a lens, will be scattered into its spectral colors (“prismatic effect”, “rainbow effect”). This effect is called dispersion and is caused by the variation of the refractive index with wavelengths.

A decrease of a medium’s refractive index with increasing wavelength is called a normal dispersion. A proportional increase of a medium’s refractive index with wavelength is called an anomalous dispersion.

Light dispersion leads to undesired chromatic aberration in an optical lens which results in blurring or smearing effect or “color fringing” around viewed objects.

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