Coherent light with an unknown wavelength passes through two very narrow slits with a separation dd $=0\mathrm{.}018$ mmmm $,$ and the interference pattern is observed on a screen a distance of L $=3\mathrm{.}00$ mm from the slits. The third$-$order bright fringe is formed at a angle of $5\mathrm{.}89$relative to the incident light.