= Elastic wave polarization
{title2=$a=a_L+a_T$}
The polarization of an <elastic wave in an isotropic solid> specifies the direction of its <displacement field> <amplitude> relative to its <wavevector>. The <longitudinal wave> component is parallel to the <wavevector>; the <transverse wave> component is perpendicular. For a real unit propagation vector $n$, $a_L=(a\cdot n)n$ and $a_T=a-a_L$. An evanescent constituent has a complex spatial <derivative> vector, so the corresponding amplitude ratio can be imaginary. The longitudinal and transverse constituents of a <Rayleigh wave> combine into <ellipse> particle motion at the free surface.
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