Answer:
At some point on say, the receiving screen, light emanating from the left side of the slit will be out of phase (a difference of 1/2 wavelengths) from light coming from the center of the slit.
Thus for every point that is left of the center of the slit, there will be a point on the right side of the slit that is out of phase,
There will be no light on the screen at that particular point and thus there will be a dark fringe there.
That is the basic explanation for the appearance of dark and bright fringes on the receiving screen.
