Richard Clark wrote:
That's not strictly true. I for one have always maintained there is
energy where it cancels, it is also somewhere else when it adds. On
the other hand if it is no where else to add, that seems to put an end
to it. Like a draw match in a tug of war, the rope may not be moving,
but its tension is obvious by the nulled energies.

This is not rocket science. If destructive interference
occurs in free space or in a transmission line, an equal
magnitude of constructive interference must occur somewhere
else in order to satisfy the conservation of energy principle.
The energy apparently"lost" during destructive interference
is simply redistributed to regions that permit constructive
interference. That is the way antenna radiation patterns
work and that is the way that waves interfere at an
impedance discontinuity in a transmission line.

http://www.mellesgriot.com/products/optics/oc_2_1.htm

"Clearly, if the wavelength of the incident light and
the thickness of the film are such that a phase difference
exists between reflections of p, then reflected wavefronts
interfere destructively, and overall reflected intensity is
a minimum. If the two reflections are of equal amplitude,
then this amplitude (and hence intensity) minimum will be
zero." (Referring to 1/4 wavelength thin films.)

"In the absence of absorption or scatter, the principle of
conservation of energy indicates all 'lost' reflected intensity
will appear as enhanced intensity in the transmitted beam.
The sum of the reflected and transmitted beam intensities is
always equal to the incident intensity. This important fact
has been confirmed experimentally."

http://micro.magnet.fsu.edu/primer/j...ons/index.html

"... when two waves of equal amplitude and wavelength that are
180-degrees ... out of phase with each other meet, they are not
actually annihilated, ... All of the photon energy present in
these waves must somehow be recovered or redistributed in a new
direction, according to the law of energy conservation ... Instead,
upon meeting, the photons are redistributed to regions that permit
constructive interference, so the effect should be considered as
a redistribution of light waves and photon energy rather than
the spontaneous construction or destruction of light."
--
73, Cecil http://www.w5dxp.com