= Thermal penetration depth
{title2=$\delta=\sqrt{2\alpha/\omega}$}
For the no-side-loss <thermal diffusion wave>, the <thermal penetration depth> $\delta=1/a=\sqrt{2\alpha/\omega}=\sqrt{\alpha P/\pi}$ is the distance over which the <amplitude> falls by $e^{-1}$ and the <wave phase> lags by one <radian>. It is a periodic counterpart of the <thermal diffusion length>, with its numerical factor fixed by this definition. A shorter period probes a shallower region; a longer period increases sensitivity to the far-end <boundary condition>.
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