Dihedral Angle

The dihedral angle is the angle between two planes. The dihedral angle between the planes

			(1)
			(2)

which have normal vectors and is simply given via the dot product of the normals,

			(3)
			(4)

The dihedral angle is therefore trivial to compute via equation (3) if the two planes are specified in Hessian normal form

(5)

for planes (Gellert et al. 1989, p. 541).

The dihedral angle between planes in a general tetrahedron is closely connected with the face areas via a generalization of the law of cosines.