A plane monochromartic light wave falls normally on a diaphragm with two narrow slist separated bya distance `d = 2.5 mm`. A fringe pattern is formed on a screen placed at a distance `l = 100 cm` behind the diaphragm. By what distance and in which direction will these fringes be dispalced when one of the slits is covered by a glass plate of thickness `h = 10mu m`?

Explanation

Extra phase difference introduced by the glass plate is
`(2pi)/(lambda) (n - 1) h`
this will cause a shift equal to `(n - 1)(h)/(lambda)` fringe widhts
i.e by `(n - 1) (h)/(lambda) xx (l lambda)/(d) = ((n - 1)hl)/(d) = 2mm`.
The friges move down if the lower slit is convered by the plate to compensate for the extre phase shift introduced by the plate.