Wave Optics

In a double slit experiment shown in figure, when light of wavelength \(400 \mathrm{~nm}\) is used, dark fringe is observed at \(P\). If \(\mathrm{D}=0.2 \mathrm{~m}\), the minimum distance between the slits \(S_1\) and \(S_2\) is _________ \(\mathrm{mm}\).