A double slit S1-S2 is illuminated by a coherent light of wavelength λ.

A double slit $\mathrm{S}_{1}-\mathrm{S}_{2}$ is illuminated by a coherent light of wavelength $\lambda$. The slits are separated by a distance $\mathrm{d}$. A plane mirror is placed in front of the double slit at a distance $D_{1}$ from it and a screen $\Sigma$ is placed behind the double slit at a distance $D_{2}$ from it. The screen $\Sigma$ receives only the light reflected by the mirror. Find the fringe-width of the interference pattern on the screen.