Two light rays having the same wavelength $\lambda $ in vacuum are in phase initially. Then the first ray travels a path ${{L_1}}$ through a medium of refractive index ${{n_1}}$ while the second ray travels a path of length ${{L_2}}$ through a medium of refractive index ${{n_2}}$. The two waves are then combined to produce interference. The two waves are then combined to produce interference. The phase difference between the two waves is