||D. A. B. Miller, "Huygen's wave
propagation principle corrected" Optics Letters 16, 1370-1372, (1991).
principle that each point on a wave front represents a source of spherical waves is
conceptually useful but is incomplete; the backward parts of the wavelets have to be
neglected ad hoc, otherwise backward waves are generated. The problem is solved
mathematically by Kirchhoff's rigorous integration of the wave equation, but the intuitive
appeal of Huygens's simple principle is lost. I show that, by using spatiotemporal dipoles
instead of spherical point sources, one can recover a simple principle of scalar wave
propagation that is correct whenever the concept of a wave front is meaningful.
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