Light of wavelength $\lambda$ in air enters a medium of refractive index $\mu$. Two points in this medium lying along the path of this light, are at a distance of $x$ apart. The phase difference between these points is 

Wavelength in the medium,                                   $\lambda \text{'}=\frac{\lambda }{\mu }$

                                                                           $∆\varphi =\left(\frac{2\pi }{\lambda \text{'}}\right)x=\frac{2\pi }{{\displaystyle \raisebox{1ex}{$\lambda $}\!\left/ \!\raisebox{-1ex}{$\mu $}\right.}}$

                                                                              $x=\frac{2\pi \mu x}{\lambda }$