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 apartThe phase difference between these point is
A. `mu((2pi)/lambda)x`
B. `1/mu((2pi)/lambda)x`
C. `(mu-1)((2pi)/lambda)x`
D. `1/(mu-1)((2pi)/lambda)x`