= Wave absorption
<Wave absorption> transfers <energy> from a propagating <wave> to other <degrees of freedom> of a medium. For a <plane wave> that is a <monochromatic wave>, with amplitude proportional to $e^{i(k'+ik'')z}$ and $k''>0$, its <wave intensity> decreases as $I(z)=I(0)e^{-2k''z}$. The intensity absorption coefficient is therefore $2k''$ when this loss is due to absorption. A decreasing transmitted <wave intensity> can also result from <wave reflection> or <wave scattering>, which redistribute <energy> rather than necessarily converting it into <heat>.
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