# Find Optical Path Difference?

**Classical Mechanics**Level 4

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 \(y\).