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 $x$ apart. The phase difference between these points is:

• A
  $\frac{2\pi \mu x}{\lambda}$
• B
  $\frac{2\pi x}{\mu \lambda}$
• C
  $\frac{2\pi (\mu - 1)x}{\lambda}$
• D
  $\frac{2\pi x}{(\mu - 1)\lambda}$