A double slitS1S2 is illuminated by a coherent light of wavelength [\lambda.] The slits are separated by a distanced. A plane mirror is placed in front of the double slit at a distanceD1from it and a screen æ is placed behind the double slit at a distanceD2 from it (see the following figure). The screen æ receives only the light reflected by the mirror. Find the fringe-width of the interference pattern on the screen.
