(a) What is meant by coherent sources. ? Why are they necessary for observing a sustained

      interference pattern?

(b) Show that the superposition of the waves originating from two coherent sources S1 and S2 

      described by y1=a cos ωt and y2=a cos ωt+ϕ at a point, produces a resultant intensity

      I=4 a2cos2 ϕ/2

      Hence write the conditions for the appearance of dark and bright fringes.          (5 marks)

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