Q .     A vessel `ABCD` of `10` cm width has two small slits `S_1 and S_2` sealed with identical glass plates of equal thickness. The distance between the slits is `0.8` mm. `POQ` is the line perpendicular to the plane `AB` and passing through `O`, the middle point of `S_1` and `S_2`. A monochromatic light source is kept at `S, 40` cm below `P` and `2` m from the vessel as shown in the figure. Now, a liquid is poured into the vessel and filled upto OQ. The central bright fringe is found to be at Q.` Calculate the refractive index of the liquid.