Eikonal function
= Eikonal function
{title2=$S$}
The rapidly varying real phase of a short-<wavelength> field written $a(\mathbf r,t)e^{iS(\mathbf r,t)}$. Its derivatives define the local <wavevector> $\mathbf k=\nabla S$ and <angular frequency> $\omega=-\partial_t S$. The <WKB approximation> assumes that $a$ and background parameters vary little across one <wavelength>.