Two identical narrow slits ${\mathrm{S}}_1$ and ${\mathrm{S}}_2$ are illuminated by the light of a wavelength $\lambda$ from a point source $\mathrm{P}$.



If, as shown in the diagram above, the light is then allowed to fall on a screen, and if $n$ is a positive integer, the condition for destructive interference at $\mathrm{Q}$ is