# Translations:Rays/9/en

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A vector represents the geometric idea of a directed line segment. A *vector* is a quantity that has direction as well as magnitude. Boldface letters usually are used to denote vectors. The vector represents a raypath as a function of traveltime *t*. The foot of this vector is at the origin (0,0), and the arrowhead is at the point on the ray-path. For ease of notation, often the functional dependence on traveltime is not written explicitly but is understood implicitly. A 2D straight ray starting from the source point and going at constant speed *v* in the direction of the constant angle (as measured from the horizontal) has the parametric representation