On Oct 13, 2010, at 4:21 PM, Jacob Keller wrote: > While we are on embarrassingly simple questions, I have wondered for a long > time what is the reference phase for reflections? I.e. a given phase of say > 45deg is 45deg relative to what? Is it the centrosymmetric phases? Or a > theoretical wave from the origin?
Conventionally, the phase difference is defined in terms of the path difference between the path traveled by a photon scattering from a point at the origin and a photon scattering from a point displaced by some finite amount (usually a multiple of a unit cell vector) from the origin. The key is the only physically meaningful quantity is the difference in path length, not the absolute values. Hence, if that difference is zero, then the phase angle will be zero. So it is compared to the wave from the origin (which is not theoretical, although by the theory we use -- the First-Order Born Approximation -- it appears to violate conservation of energy. Fortunately, the beamstop prevents us from having to face that horrific reality.) -- Bill
