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 $e^{i(k^{\prime }+i{k}^{\prime \prime })z}$ and $k^{\prime \prime }>0$, its wave intensity decreases as $I(z)=I(0)e^{-2k^{\prime \prime }z}$. The intensity absorption coefficient is therefore $2k^{\prime \prime }$ 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.