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 m towards the slits, the change in fringe width is 3 $\times$ 10^$-$3 cm. If the distance between the slits is 1 mm, then the wavelength of the light will be ____________ nm.