(1) If we use two sodium vapour lamps to illuminate two pinholes we will not observe any interference fringes. This is because of the fact that the light wave emitted from an ordinary source (like a sodium lamp) undergoes abrupt phase changes in times of the order of 10–10 seconds. Thus the light waves coming out from two independent sources of light will not have any fixed phase relationship and would be incoherent, when this happens, the intensities on the screen will add up.
(2) British physicist Thomas Young used an ingenious technique to “lock” the phases of the waves emanating from S1 and S2. He made two pinholes S1 and S2 (very close to each other) on an opaque screen Fig.(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. Thus, the two sources S1 and S2 will be locked in phase; i.e., they will be coherent.
Thus spherical waves emanating from S1 and S2 will produce interference fringes on the screen GG¢, as shown in Fig. (b).
We should mention here that the fringes are straight lines although S1 and S2 are point sources. If we had slits instead of the point sources Fig., each pair of points would have produced straight line fringes resulting in straight line fringes with increased intensities.
