A double slit S1 - S2 is illuminated by a coherent light of wavelength λ. The slits are separated by a distance d. A plane mirror is placed in front of the double slit, at a distance D1 from it and a screen ∑ is placed behind the double slit at a distance D2 from it (figure 17-E2). The screen ∑ receives only the light reflected by the mirror. Find the fringe-width of the interference pattern on the screen.


In this problem, the rays after passing through the slit fall on the mirror. So both rays will be shifted by and hence will form a normal interference pattern. But the sources would be the virtual image of slits and .

So the distance of the screen from the virtual images of and is


therefore, fringe width is


1