A thin plastic sheet of refractive index $\text{1.6}$ is used to cover one of the slits, among the two slits, in a double-slit arrangement. The central point on the screen is now occupied by what would have been the $7^{th}$ bright fringe before the thin plastic sheet was used. If the wavelength of light is $600 \mathrm{nm}$, the thickness (in $\mathrm{\mu m}$) of the thin plastic sheet is