A vessel $\text{ABCD}$ of $\text{10 cm}$ width has two small slits ${\text{S}}_1$ and ${\text{S}}_2$  sealed with identical glass plates of equal thickness. The distance between the slits is $\text{0}\text{.8 mm}$. $\text{POQ}$ is the line perpendicular to the plane $\text{AB}$ and passing through $\text{O}$, the



middle point of ${\text{S}}_1$ and ${\text{S}}_2$, A monochromatic light source  is kept at $\text{S}$, $\text{40 cm}$ cm below $\text{P}$ and $\text{2 m}$ from the vessel, to illuminate the slits as shown in the figure alongside. After a liquid is poured into the vessel and filled upto $\text{OQ,}$ it was found that the central bright fringe is now located at $\text{Q}$. Calculate the refractive index of that liquid.