Interfacial gravity-wave dispersion relation
= Interfacial gravity-wave dispersion relation
For two inviscid layers of equal depth $h$ between rigid lids, with lower density $\rho_2$ and upper density $\rho_1$, a horizontal mode of wavenumber $k$ satisfies
$$
\omega^2=\frac{\rho_2-\rho_1}{\rho_1+\rho_2}gk\tanh(kh).
$$
The sign reverses when the heavier fluid is above, producing the <Rayleigh-Taylor instability>.