High frequency approximation

The ray theoretic ansatz assumes that $K\approx k=\omega s$, which involves setting the contribution of $\gamma$ and its derivates to zero. Thus, the frequency dependence of the ray path equation is,

$$\frac{\partial}{\partial \omega}\left(\nabla K\right) = 0,$$ (13)

which requires that the ray paths are stationary with respect to frequency.