 Solution

Let $\theta$ be the angle made by the group velocity ray with the horizontal. The internal gravity wave dispersion relation givesEach sawtooth face changes height by $2h_0$ over horizontal distance ${\lambda }_T/2$, so its slope magnitude isThe internal-wave slope criticality criterion is $s_T<\tan \theta$. Hence reflection is subcritical forIn the corresponding internal-wave ray tracing sketch, every incident ray meets one planar face and leaves it into the fluid at the same angle $\theta$ to the horizontal. The reflected ray is steeper than either face, so it clears the sawtooth rather than running into an adjacent corner.