Two coherent sources S1 and S2 produce interference fringes. If a thin mica plate is introduced in the path of light from S1 then the central maximum

Length optical path which is equivalent to length l1= optical path of ray (1)=S1a+μ(ab)+bp.l2= optical path of ray (2)=S2PBut         P will be the position of central maximum. If l1=l2.This is possible only when P moves on the side of S1 which is covered.