  The document covers the fundamental algorithm of backward propagation from
the point of view of reconstructing the wavefield captured by a "screen" in an
imaging system. Owing to a property of the Helmholtz equation, wavefields have
an inherent propagation symmetry which can be exploited for image formation. In
traditional presentations of this topic the link between this "reciprocity" and
the reconstruction procedure are motivated only at an intuitive level. I
propose to make this more rigorous by using a technique called "plane-to-plane"
propagation which is also known as "beam propagation method" in more advanced
settings.
