Young's double slit experiment is made in a liquid. The tenth bright fringe in liquid lies in screen where 6th dark fringe lies in vacuum. The refractive index of the liquid is approximately

Explanation

Fringe width `beta=(lambdaD)/(d)`. When the apparatus is immersed in a liquid, `lambda` and hence `beta` is reduced `mu`(refractive index) times.
`10beta'=(5.5)beta or 10 lambda'((D)/(d))= (5.5) (lambdaD)/(d)`
or `(lambda)/(lambda')=(10)/(5.5)= mu or mu=1.8`