(i) Explain why introducing a thin transparent sheet in front of one slit of a Young’s double-slit experiment causes the entire fringe system to shift on the screen even though the fringe width remains unchanged.
(ii) Describe how, in the same experiment, changing the medium surrounding both the slits and the screen (for example from air to a liquid) affects (a) the fringe width and (b) the amount of fringe shift produced by a transparent sheet placed before one slit.