The diagram, given here, shows several fringes, due to double slit interference/contained' in a broad diffraction peak. When the separation between the slits is large compared to their width, the diffraction pattern becomes very flat and we observe the two slit interference pattern. 1
[Note : The students may be awarded 1 mark even if they just draw the diagram.Two basic features :
(i) The interference pattern has a number of equally
spaced bright and dark bands while diffraction pattern has a central bright maximum which is twice as wide as the other maxima.
(ii) Interference pattern is the superimposition of two waves slits originating from two narrow slits. The diffraction pattern is a superposition of a continuous family of waves originating from each point on a single slit.
(iii) For a single slit of width V the first null of diffraction pattern occurs at an angle of Lanbda/A. At the
same angle of Lambda/a, we get a maxima for two narrow slits separated by a distance a.