Cecil Moore wrote:

"PFtotal = P1 + P2 - 2*SQRT(P1)*SQRT(P2)"

so if you don't like negative power terms, you should confront both
Eugene Hecht and Dr. Best.

Don't be foolish. Obviously neither of the P terms can be negative if
PFtotal is supposed to represent a real number. I have no issues to
confront with either of the gentlemen. It is where you diverge from
Hecht (and Maxwell, and Born and Wolfe, and Jackson) that I take issue.

Nor will we find a negative scalar quantity accompanied by the claim
that the negative sign indicates a change in direction, as you have done.


On the contrary, in equation 9.16 above, according to Hecht, the
interference term is negative indicating "total destructive
interference", his words, not mine. Here's Hecht's quote from _Optics_.

One statement does not contradict the other, as the subject in each
sentence is entirely different. Both statements are obviously true.

Similarly, power and irradiance do not physically propagate and they
do not physically interact.


On the contrary, they do physically interact for coherent waves as can
be inferred by the interference equations. Please reference Chapter 9
in _Optics_, by Hecht. The mathematical interaction of power and
irradiance is a *result* of superposition of coherent EM waves. That's
where the interference equations involving irradiance come from.

As I explained they come from the fact that the fields interact, and
that power and intensity (or irradiance) go as the square of the field.
Lets say the square of F1 (field 1) is proportional with P1, and the
square of F2 is proportional with P2. And lets say the mathematical
description of the way the two fields interact is as follows:
Ftotal = (F1 - F2)*(F1 - F2). (Looks kinda like modulation, but I
digress.) We can then write that as
Ftotal = F1^2 + F2^2 - 2*F1*F2. By substitution then,
PFtotal = P1 + P2 - 2*SQRT(P1*P2). And that's where your favorite
equation comes from.

73, ac6xg