Wave ray
= Wave ray
{title2=$d\mathbf x/dt=\nabla_{\mathbf k}\omega$}
A path followed by a slowly modulated <wave packet> under a local <dispersion relation>. Its <velocity> is the <group velocity>, while its <wavevector> evolves as $d\mathbf k/dt=-\nabla_{\mathbf x}\omega$. In a uniform stationary medium both are constant, so the ray is straight even when <phase velocity> and <group velocity> differ.