In Young's double-slit experiment, fringes of width $\beta$ are produced on a screen kept at a distance of 1 m from the slits. When the screen is moved away by $5 \times 10^{-2}$ m, fringe width changes by $3 \times 10^{-5}$ m. The separation between the slits is $1 \times 10^{-3}$ m. The wavelength of the light used is __________ nm.