I have an online homework question and my classmate told me the answer but I would really like someone to explain to me how that answer was determined. I do not understand. Q: Consider the electric field observed at a point O that is far from the two slits, say at a distance r from the midpoint of the segment connecting the slits, at an angle θ from the x axis. Here, far means that r≫d, a regime sometimes called Fraunhofer diffraction. The critical point is that the distances from the slits to point O 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) of the wave from the lower slit that arrives at point O. EQUATIONS: E(x,t)=Eleftcos[2π/λ(x−ct)]. Φ(x,t)=kx−ωt ω=2πf k=2π/λ SOLUTION: Φlower(O, t) = 2π/λ (r−ct) + π/λ dsin(θ) I just really don't understand how that is derived... can someone please help explain?