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12.
Finding average dispersion value and Abbe number
1. Dispersion value
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Refractive Index varies depending on color (wavelength) of the light. Therefore, the Abbe
Refractometer uses D-line (wavelength is 589.3nm, orange color) as the standard light
source for carrying out measurements stably and accurately. Dispersion value means the
degree that a Refractive Index changes with other light sources of difference wavelengths,
and it is expressed in an average dispersion value (nF-nC), where nF-nC is a difference
between the Refractive Index with F-line (486.1nm) and that with C-line (656.3nm).
If the peculiar value of nF-nC of a substance is known, it greatly helps to know the
characteristic of the substance, because the nF-nC of a substance is determined by its
molecular structure.
As a simple example, assume that there are two liquids that are completely different in
quality from each other but the same in Refractive Index (nD). The two liquids cannot be
discriminated from each other by the Refractive Index, but can be discrimineted by the
average dispersion value which is different from each other. Therefore, dispersion value is
very useful to identify unknown substances.
The Abbe Refractometer NAR-2T is capable of measuring average dispersion value
(nF-nC). For details, refer to page 38.
However, the NAR-2T is incapable of measuring Refractive Index with other light
sources of different wavelengths such as nF and nC.
