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 Thus, the two sources S1
and S2 will be locked in phase; i e , they will be coherent like the two
vibrating needle in our water wave example [Fig