# Translations:Ray equation/3/en

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In this section, the *position vector* **r** always represents a point on a specific raypath and not any arbitrary point in space. As time increases, **r** traces out the particular raypath in question. The seismic ray at any given point follows the direction of the gradient of the traveltime field . As before, let **u** be the unit vector along the ray. The ray in general will follow a curved path, and the *raypath vector* will be tangent to this curved raypath.