(i) What are coherent sources? Why are they necessary for observing a sustained interference pattern? How are the two coherent sources obtained in the Young's double slit experiment?
(ii) Show that the superposition of the waves originating from the two coherent sources, `S_1 and S_2` having displacement,
`Y_1 = a cosomegat and Y_2 = a cos(omegat+phi)` at a point produce a resultant intensity,
`I = 4a^2 cos^2 phi//2` Hence, write the conditions for the appearance of dark and bright fringes.