= Elastic ray
{title2=$d\mathbf x/ds=\mathbf n,\quad dt/ds=1/c$}
In a smooth isotropic medium, an <elastic ray> is the path followed by a local <wavefront> normal, with the appropriate P or S <wave speed> $c$. Arclength $s$ gives the displayed trajectory and travel-time equations. Its neighboring ray paths determine geometrical spreading, while the <eikonal equation> determines the normals. A <caustic> occurs where the neighboring paths focus and the elementary ray <wave amplitude> description ceases to be uniform.
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