When you superpose two 100w coherent laser beams, the resultant
power is indeed 400w

This is correct if the two beams are coherent and have the
same polarization. Very hard to do at optical frequencies, much
easier at lower frequencies.

Hi Tor,

We've seen the math pencil-whipped both ways now to cover all the
available answers. The devil's in the details that are not found in:
Itot = I1 + I2 + 2*Sqrt(I1*I2)cos(theta)
not to mention the glaring mistakes of the first posting of this
formula.

So? Superposition works. With a yagi antenna, through superposition you
get an EM wave which has larger intensity in certain directions than
for a single dipole with the same power. Someone far away can't tell
the difference between switching to a yagi and turning on a linear.

What the formula doesn't say is that in any real system, the wave
must have a finite extent (not be a infinite plane wave). Then there
must be destructive interference in some directions. So there isn't
a problem with conservation of energy.

Tor
N4OGW