= Right circular conical surface
{title2=$\widehat{\mathbf n}\cdot(\mathbf x-\mathbf a)=|\mathbf x-\mathbf a|\cos\beta$}
This <surface> consists of rays from a vertex $\mathbf a$ whose directions make a fixed angle $\beta$ with a unit axis <vector> $\widehat{\mathbf n}$. The displayed equation follows from the <dot product> formula for that angle. For $0<\beta<\pi$, $\beta\ne\pi/2$, it gives one circular nappe; $\beta=\pi/2$ gives a <plane>, and $\beta=0,\pi$ gives a degenerate ray. Squaring the equation produces both nappes, so the unsquared sign distinguishes them. <Generalized angular momentum in a radial monopole field> confines particle directions to such a surface.
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