YDSE is carried out in a liquid medium of refractive index $\mu =1.3$ and a thin film of air is formed in front of the lower slit as shown in the figure. If a maxima of third order is formed at the origin O (at perpendicular bisector of slits), find the thickness of the air film (in $\mu m$). “Wavelength of light in air is 0.78$\mu m$ ”