# 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 $$y$$.

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