question_answer

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 median of refractive index\[{{n}_{2}}\].   The two waves are then combined to observe interference. The phase difference between the two waves is

A)  \[\frac{2\pi }{\lambda }\left( {{l}_{2}}-{{l}_{1}} \right)\]

B)  \[\frac{2\pi }{\lambda }\left( {{n}_{1}}{{l}_{1}}-{{n}_{2}}{{l}_{2}} \right)\]

C)  \[\frac{2\pi }{\lambda }\left( {{n}_{2}}{{l}_{2}}-{{n}_{1}}{{l}_{1}} \right)\]

D)  \[\frac{2\pi }{\lambda }\left( \frac{{{l}_{1}}}{{{n}_{1}}}-\frac{{{l}_{2}}}{{{n}_{2}}} \right)\]