Translations:Ray equation/10/en

The slowness $n\left(x,y\right)$ 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 $\partial n/\partial x$ is called the partial derivative of slowness with respect to x. The partial derivative $\partial n/\partial y$ is defined similarly. The gradient of the slowness surface $n\left(x,y\right)$ is the vector with these partial derivatives as components; that is,