Gene Fuller wrote:
Why do you attribute such magic to the word "interference"? Do you think
that Hecht's "interaction" is any different than superposition?

It is not magic. "Interference" and "superposition" simply
have different definitions.

Interference is a subset of superposition, i.e. interference
cannot occur without superposition but superposition can occur
without interference. This subject is covered in every optics
text that I have ever seen, including Born and Wolf. Given two
waves of equal power densities (irradiances) if the resultant
irradiance is not equal to the sum of the two irradiances, then
interference has occurred.

What if the waves are not quite anti-parallel, say at an angle of 179
degrees? Is interference now possible?

Impossible in a transmission line which is the context.
In free space, I would guess that interference is possible
in their common direction of travel.

Suppose the waves are only 1 degree from parallel. Does that negate the
interference?

For coherent waves in free space, that would ensure interference
until the beams diverged. It should result in the usual light
and dark interference rings.

Repeating: This is a distinction with no technical value.

Maybe it would help if you published a video of you waving
your hands as you scream that assertion at the top of your
lungs? :-)
--
73, Cecil http://www.w5dxp.com