* If you look *at the shadow cast by an object, you will see, if you look closely enough, variations in brightness
in the shadow. These variations of light and dark cannot be explained by assuming
a straight line propagation of light rays.
This phenomenon of "diffractio", or diffraction of light, as we now call it, was first mentioned by
Francesco Grimaldi in the 17th century. (Here
is a beautiful example of such a diffraction pattern.)

The basic equations that describe diffraction, while easy to
write down, are notoriously difficult to solve exactly, and exact closed form solutions
are rare. In the late 1950s, the Russian
scientist Ufimtsev
developed and expanded the earlier work of H.M. MacDonald (cf.
Electric Waves and also
here where MacDonald's Physical Optics was first introduced), and developed a powerful *
asymptotic *
method for solving high-frequency diffraction problems.

This new method was particularly suited to stealth
design, since it enabled the far field diffraction pattern of *radar* waves
from airplanes to be calculated. In particular, it was now possible to calculate
what shapes would NOT reflect the radar waves back to their source, and hence would
be undectable by radar.

Surprisingly, Ufimstev's work was all but ignored in the Soviet
Union, but not so in the US. After the US Air Force had translated his work from
the original Russian, it was read by Denys Overholser, a radar specialist who
realized its practical implications...and in fact, in a matter of months after he
had read the paper, he had translated it into a computer program, ready for stealth
design at the fabled "Skunk
Works" shop. Both the Lockheed F-117
Stealth Fighter and the Northrop B-2 Stealth Bomber were designed with the aid of
Ufimtsev's approach.