# Translations:Ray equation/10/en

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The slowness is a scalar function that depends on coordinates *x* and *y*. However, we can hold *y* constant and consider the curve that gives the variation of slowness with *x* alone. The slope of this curve is called the partial derivative of slowness with respect to *x*. The partial derivative is defined similarly. The gradient of the slowness surface is the vector with these partial derivatives as components; that is,