
Refractive Index, Abbe Value and Dispersion Formula 

The optical properties are defined by two primary parameters, the refractive index and Abbe number. The refractive index is a measure of the bending power of a light beam when passing from one medium into another. The Abbe or vnumber named after German physicist Ernst Abbe, is a measure of the dispersive power and is defined as:
Where n_{d}, n_{F} and n_{C} are the refractive indices of the material at wavelengths of the Fraunhofer d, F and C spectral lines (587.6 nm, 486.1 nm and 656.3 nm respectively). Besides , the dispersion based on eline is often used as:
Table 1 shows some basic information of glass code, two primary optical parameters and density. Table 2 shows spectral lines and designated letters for them. The refractive indices of optical glasses are given at these wavelengths with the NBK7 as an example.
The refractive index at a wavelength other than the spectral lines can be calculated from a dispersion formula. SCHOTT uses Sellmeier dispersion formula:
where λ is the wavelength in um ,and B_{1}, B_{2}, B_{3}, C_{1}, C_{2} and C_{3 }are coefficients to be determined for each optical glass type. Table 3 gives the parameters of NBK7 and NSF11. HOYA’s dispersion formula is derived from a series expansion of the theoretical formula:
where λ is the wavelength in um ,and A_{0}, A_{1}, A_{2}, A_{3}, A_{4} and A_{5 }are coefficients. The accuracy of both formulas for calculating refractive index at a wavelength in the visible and near infrared range has an order of 10^{6}.
Properties:

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