If Young’s double slit apparatus is shifted from air to water, then

When Young's double-slit set up for interference is shifted from air to within water then the fringe width decreases because the refractive index of water is more than that of the air.
Originally the fringe width is given by

$\mathrm{\beta }=\frac{\mathrm{D\lambda }}{\mathrm{d}}; and \lambda \propto \frac{1}{\mu }$; ${\beta }^{\text{'}}=\frac{\beta }{\mu }$