Two light waves having the same wavelength \(\lambda\) in vacuum are in phase initially. Then the first wave travels a path \(L _{1}\) through a medium of refractive index \(n_{1}\) while the second wave travels a path of length \(L_{2}\) through a medium of refractive index \(n _{2}\). After this the phase difference between the two waves is:

1. A \(\frac{2 \pi}{\lambda}\left( n _{1} L _{1}- n _{2} L _{2}\right)\)
2. B \(\frac{2 \pi}{\lambda}\left(\frac{ L _{2}}{ n _{1}}-\frac{ L _{1}}{ n _{2}}\right)\)
3. C \(\frac{2 \pi}{\lambda}\left(\frac{ L _{1}}{ n _{1}}-\frac{ L _{2}}{ n _{2}}\right)\)
4. D \(\frac{2 \pi}{\lambda}\left( n _{2} L _{1}- n _{1} L _{2}\right)\)