= Dispersion relation for interfacial gravity waves between rigid boundaries
{title2=$\omega^2=g(\rho_2-\rho_1)|k|/[\rho_1\coth(|k|h_1)+\rho_2\coth(|k|h_2)]$}
For two incompressible inviscid layers at rest between rigid horizontal boundaries, with the heavier fluid below and no surface tension, the displayed <dispersion relation> follows from <Laplace equation> for the velocity potentials, no normal flow at the walls, equal normal velocities at the interface, and pressure continuity. The positive density contrast supplies the restoring force. In the long-wave limit the squared wave speed is $g(\rho_2-\rho_1)/(\rho_1/h_1+\rho_2/h_2)$.
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