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.