Sellmeier Dispersion fitting

refractive index data fitting by using nlinfit function and Sellmeier Dispersion equation.
load your refractive index data file here in my example Nb2O5 'you can change this file'. put your expected values of sellmeier coefficients
coeffs1 = [1 1.8 0.1 1 0.12];
in selmier.m file.
you can change the number of coefficients by example:
coeffs1 = [1 1.8 0.1 1 0.12 5 18];
and get as a result the fitted values in newcoeff.