Keith Dysart wrote:
More curiousity: Can P1 and P2 have different signs, that is,
the power is going in different directions?

Please reference Chapter 9, Interference, in "Optics",
by Hecht, 4th edition. The two interfering waves are
traveling in the same direction. The associated
powers exist together at a point of interference.
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73, Cecil http://www.w5dxp.com