Now consider the electric field observed at a point OOO that is far from the two slits, say at a distance rrr from the midpoint of the segment connecting the slits, at an angle θθtheta from the x axis. Here, far means that r≫dr≫d, a regime sometimes called Fraunhofer diffraction. The critical point is that the distances from the slits to point OOO are not equal; hence the waves will be out of phase due to the longer distance traveled by the wave from one slit relative to the other. Calculate the phase Φlower(O, t)Φlower(O, t) of the wave from the lower slit that arrives at point OOO.