Let (x ′, y ′, 0) be a point inside the slit over which it is being integrated. If (x, 0, z) is the location at which the intensity of the diffraction pattern is being computed, the slit extends from x ′ = − − a / 2 to + a / 2 , and from y ′ = − − ∞ ∞ to ∞ ∞ .