In a double slit experiment shown in figure, when light of wavelength 400 nm is used, dark fringe is observed at P. If D =0.2 m, the minimum distance…

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