The diffraction or migration equation
could be called the ``wavefront healing'' equation.
It smooths
back together any lateral breaks in the
wavefront that may have been caused
by initial conditions or by the lens term.
The 15 migration equation has the same
mathematical form as the heat-flow equation.
But the heat-flow equation has all real numbers,
and its physical behavior is more comprehensible.
This makes it a worthwhile detour.
A two-sentence derivation of it follows.
(1) The heat
flow *H*_{x} in the *x*-direction equals the
negative of the gradient of
temperature *T* times the heat conductivity .(2) The decrease of temperature is
proportional to the divergence
of the heat flow divided by
the heat storage capacity *C* of the
material.
Combining these, extending from one
dimension to two, taking constant and ,gives the equation

(1) |

10/31/1997