Monochromatic light of wavelength $500 \mathrm{nm}$ is incident on two parallel slits separated by a distance of $5\times {10}^{-4} \mathrm{m}$. The interference pattern is obtained on a screen at a distance of $1.0 \mathrm{m}$ from the slits. The intensity of the central maximum is $I_0$. When one of the slits is covered by a glass sheet of thickness $5\times {10}^{-6} \mathrm{m}$ and refractive index 1.5, the lateral shift of the central maximum is …….mm.