what happens when slit is inserted in front of one of the slits in young double slit experiment

When plate is inserted in front of one of the slits then fringes pattern shift above or below depending on which slit you insert the glass plate ..fringe shift is equal to D÷d(u-1)t where t is thickness of plate

When the slit is inserted then the fringe pattern shifts ....if the slit width is t distance between slit is d and distance between slit and screen isD and u is the refractive ndex then fringe shift is equ to D(u-1)t÷d

How did you get this formula please give the derivation as the book I am studying has not given the proof of this fringe shift....