In a double slit experiment with monochromatic light, fringes are obtained on a screen placed at some distance from the plane of slits. If the screen is moved by $5\times {10}^{-2} \mathrm{m}$ towards the slits, the change in fringe width is $3\times {10}^{-3} \mathrm{cm}$. If the distance between the slits is $1 \mathrm{mm}$, then the wavelength of the light will be____$\mathrm{nm}$.