12(a)] These were illuminated by another pinholes that was in turn, lit by a
bright source Light waves spread out from S and fall on both S1 and S2 S1 and S2 then behave like two coherent sources because light waves
coming out from S1 and S2 are derived from the same original source
and any abrupt phase change in S will manifest in exactly similar phase
changes in the light coming out from S1 and S2