Two light waves having the same wavelength $\lambda$ in vacuum are in phase initially. Then the first wave travels a path L₁ through a medium of refractive index n₁ while the second wave travels a path of length L₂ through a medium of refractive index n₂ . After this the phase difference between the two waves is :

1. A ${{2\pi } \over \lambda }\left( {{n_1}{L_1} - {n_2}{L_2}} \right)$ Correct answer
2. B ${{2\pi } \over \lambda }\left( {{n_2}{L_1} - {n_1}{L_2}} \right)$
3. C $${{2\pi } \over \lambda }\left( {{{{L_1}} \over {{n_1}}} - {{{L_2}} \over {{n_2}}}} \right)$$
4. D $${{2\pi } \over \lambda }\left( {{{{L_2}} \over {{n_1}}} - {{{L_1}} \over {{n_2}}}} \right)$$