Question

will their be a shift in fringe pattern when both the slits in YDSE are covered by a glass slab of refractive index 'U' ? also give a valid reason to the question

Answer 1

Nothing spectacular indeed, only a half of the beam will pass. The intensity of wave that passed through the slits directly depends on the size of these openings. How this energy is distributed later depends on interference etc.
Hope it helps