Normal line
= Normal line
{title2=$\mathbf x=\mathbf p+t\mathbf n$}
At a <point> $\mathbf p$ of a smooth <surface> with nonzero <normal vector> $\mathbf n$, its normal line is $\{\mathbf p+t\mathbf n:t\in\mathbb R\}$. For a <level set> $F=c$ with $\nabla F(\mathbf p)\ne0$, one can use $\mathbf n=\nabla F(\mathbf p)$.