Most imaging schemes involve characterizing the interaction of an electromagnetic wave with a target medium. This interaction can be generally described by the wave equation: This equation relates the spatial variation (Laplacian) of the field to the electrical permittivity, e, and magnetic permeability, ix, of the medium. The latter quantities account for the induced alignment of the electrical dipole (polarization) and magnetic moment (magnetization) in the material by the propagating field, and they determine c, the speed of light, which is equal to 1/ Eµ . For a time-harmonic source, equation 1 reduces to the Helmholtz equation: where k, the wave number, is equal to wic, and w is the wave frequency in radians-s-1.
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