= Seismic ray
A <seismic ray> is a trajectory describing high-frequency seismic <energy> propagation in a medium whose properties vary slowly compared with the <wavelength>. In an isotropic medium it is normal to local travel-time wavefronts, and its <travel time> is $T=\int ds/v$ along the path.
Changes in <wave speed> bend rays, while interfaces can reflect or refract them. The approximation is useful for interpreting arrival times, but <diffraction>, <interference> and caustics require more than a single-ray description. In anisotropic media, <energy> propagation and the wavefront-normal direction need not coincide.
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