Value of the constant in 'variation of refractive index'

[Donchay]

In optics, given the below formula

n_λ= 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 n_λ decreases.

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

 

[DrDu]

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

 

[Donchay]

Thank You DrDu.
While the above formula is for Optical Dispersion, then is it the same dispersion for the Abbe Number:

V_d=(n_D-1)/n_F-n_C

where n_F-n_C is the dispersion according to this link http://glassproperties.com/abbe_number/

but somehow on other link I read that n_F-n_C is called Principal Dispersion. I try to search on more about Principal Dispersion but there is almost none explanation about it.

 

[DrDu]

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.

 

[Donchay]

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

 

[DrDu]

I would try something like $n(\lambda)=n_C+(n_F-n_C)\frac{1/\lambda_C^2-1/\lambda^2}{1/\lambda_C^2-1/\lambda_F^2}$.