## Value of the constant in 'variation of refractive index'

In optics, given the below formula

nλ= A + B/λ2 + C/λ4 +......

where A, B and C are constants.

From the above relationship we can deduce that as the wavelength λ increases, the variation of refractive index nλ decreases.

How do we measure the constant value of A,B and C at the first place?

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 Recognitions: Science Advisor You measure the refractive index at several wavelengths and then you fit the formula to the values obtained.
 Thank You DrDu. While the above formula is for Optical Dispersion, then is it the same dispersion for the Abbe Number: Vd=(nD-1)/nF-nC where nF-nC is the dispersion according to this link http://glassproperties.com/abbe_number/ but somehow on other link I read that nF-nC is called Principal Dispersion. I try to search on more about Principal Dispersion but there is almost none explanation about it.

Recognitions:

## Value of the constant in 'variation of refractive index'

n_F is the index of refraction for blue light while n_C is for red light (see the table with the line frequencies).
One assumes that for glass the dispersion is approximately linear over the optical frequency range.

 I understand about the line frequencies. Just that is it n(lambda)=n_F - n_C ?
 Recognitions: Science Advisor I would try something like ## n(\lambda)=n_C+(n_F-n_C)\frac{1/\lambda_C^2-1/\lambda^2}{1/\lambda_C^2-1/\lambda_F^2} ##.

 Tags glass, optics, refractive index