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

1. ### Donchay

10
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?

2. ### DrDu

4,165
You measure the refractive index at several wavelengths and then you fit the formula to the values obtained.

3. ### Donchay

10
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.

4. ### DrDu

4,165
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.

5. ### Donchay

10
I understand about the line frequencies. Just that is it n(lambda)=n_F - n_C ?

6. ### DrDu

4,165
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} ##.