How does refractive index ($\mu$) of a material vary with respect to wavelength (l$\lambda$) ? A and B are constants

$\mu = A + B\lambda^2$ $\mu = A + \frac{B}{\lambda^2}$ $\mu = A + B\lambda$ $\mu = A + \frac{B}{\lambda}$

$\mu = A + \frac{B}{\lambda^2}$

$\text{Cauchy relation } \mu = A + \frac{B}{\lambda^2}$