= Thermal radiation between parallel conducting plates
{title2=$u(\omega)=\hbar\omega^2/[2\pi c^2d(e^{\hbar\omega/(k_{\rm B}T)}-1)]$}
Two wide perfect-conductor plates separated by $d$ support a gapless <transverse electromagnetic mode> whose <electric field> is normal to the plates. At $\omega\ll c/d$, multiply the <single-polarization two-dimensional photon density of states> by $\hbar\omega$ and the <Bose-Einstein distribution> to get the title's spectrum. This is a low-frequency statement; it need not describe the full thermal spectrum if thermally populated higher branches cross their cutoffs.
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