If the slit lies in the x′-y′ plane, with its center at the origin, then it can be assumed that diffraction generates a complex wave ψ, traveling radially in the r direction away from the slit, and this is given by: Ψ Ψ = ∫ ∫ s l i t i r λ λ Ψ Ψ ′ ′ e − − i k r d s l i t