You should notice, what happens to the strength distribution on screen when the light is allowed to go through two narrow slits close to each other (Fig.A). In Young's double slit experiment we have seen that it arrangement will give an interference pattern comprising alternate dark and bright fringes on screen. However, in that experiment, we have assumed each of slit as behaving like point source (we did not consider the width of slit). This was equivalent to the condition that. Now from our earlier discussion in this chapter, we have seen that if, the ray bends around the sharp edge but every where (within the area of view) the waves superpose constructively and we see illumination all over.(No diffraction pattern due to either slit)