Contact Angle

$\begin{figure}\begin{center}\BoxedEPSF{ContactAngle.epsf}\end{center}\end{figure}$

The Angle $\alpha$ between the normal vector of a Sphere (or other geometric object) at a point where a Plane is tangent to it and the normal vector of the plane. In the above figure,

$\displaystyle \alpha$	$\textstyle =$	$\displaystyle \cos^{-1}\left({a\over R}\right)$
	$\textstyle =$	$\displaystyle \sin^{-1}\left({R-h\over R}\right).$

See also Spherical Cap