Find Optical Path Difference?

If 2 parallel rays of light are travelling in a vacuum parallel to the $x$ -axis, with one of them passes through a medium with refractive index varying with $x$ from $x = 1$ to $x=2$ and $\mu = \dfrac{x^2(x^2+4)}{x^4+3x^2+1}$ and the other moves in vacuum, after the first ray comes again to vacuum. Let the optical path difference between them be of the form of $\text{arccot } y$ , find $y$ .