Since we are for the moment only interested in the amplitude and relative phase, we can ignore any overall phase factors that are not dependent on x or n . We approximate ( x − − n a ) 2 + L 2 ≈ ≈ L + ( x − − n a ) 2 / 2 L . In the Fraunhofer limit we can neglect terms of order a 2 2 L in the exponential, and any terms involving a / L or x / L in the denominator. The sum becomes Ψ Ψ = A e i ( k ( x 2 2 L + L ) − − ω ω t + ϕ ϕ ) L ∑ ∑ n = 0 N − − 1 e − − i k x n a L