In the far field, wherein the parallel rays approximation can be employed, the Green's function, ψ ψ ( r | r ′ ) = e i k | r − − r ′ | 4 π π | r − − r ′ | , simplifies to ψ ψ ( r | r ′ ) = e i k r 4 π π r e − − i k ( r ′ ⋅ ⋅ r ^ ^ ) as can be seen in the adjacent figure.