The Cauchy’s dispersion formula is 

Cauchy's transmission equation is an empirical relationship between a transparent material's refractive index and wavelength of light in optics. It was named after Augustin-Louis Cauchy, a mathematician who described it in 1836
Cauchy's equation in its most general form is    $n\left(\lambda \right)=A+\frac{B}{{\lambda }^2}+\frac{C}{{\lambda }^4}+\dots$