The symmetry axes of an object are lines about which it can be rotated through some angle which brings the object to a new orientation which appears identical to its starting position. An axis is called n-fold if the smallest angle which brings the object back to its original appearance is 360/n degrees. For example, a plane pentagon has a single 5-fold (72 degree) axis of symmetry and five 2-fold (180 degree) axes of symmetry. The 5-fold axis is orthogonal to its plane, while the five 2-fold axes each lie in the plane and pass through one of the vertices and the opposite edge midpoint. All the symmetry axes of a polyhedron necessarily intersect at a common point at the center of the object.
Part of the beauty of polyhedra lies in their many axes of symmetry. Every uniform polyhdron (regular or semiregular) falls into one of the following five classes according to where its symmetry axes lie. Be sure to examine the models listed here and identify all their symmetry axes. You can create your own models of axes of symmetry with the cylinder intersecter. A great toy to help one understand symmetry axes is the Zometool.
The cube contains three different types of symmetry axes:
Here is a model of the cube displaying all of its symmetry axes. The model also includes the octahedron which is dual to the cube, and so shows that the cube and the octahedron possess the same symmetry. In the case of the octahedron the same thirteen axes appear as:
Some other polyhedra with the same thirteen symmetry axes are the cuboctahedron (in which the 2-fold axes pass through the vertices) and the rhombicuboctahedron (in which each axis goes through the center of two of the 26 faces).Yet another is the snub cube. (Find its axes!) However, the snub cube is chiral, lacking any planes of mirror symmetry. So the axes of symmetry are not enough to completely characterize the symmetry of a polyhedron; they do not distinguish between chiral and reflexible polyhedra.
In the case of the dodecahedron the same thirty-one axes appear as:
The tetrahedron has only two different types of axes, as this model of the tetrahedron and its seven axes of symmetry displays:
Because the tetrahedron (unlike the other polyhedra above) has faces opposite vertices (rather than faces opposite faces and vertices opposite vertices), it does not have three kinds of symmetry axes.
If we place a tetrahedron inside a cube, we see how the symmetry axes of the tetrahedron relate to the symmetry axes of the cube:
We can also place a tetrahedron inside a dodecahedron and see that every symmetry axis of the tetrahedron is an axis of the same type for the dodecahedron.
An n-gonal prism or antiprism also has only two kinds of axis of symmetry as this pentagonal prism model shows:
Prism symmetry is called dihedral symmetry. If n is odd each 2-fold axis goes through one face and one opposite edge. If n is even, half the 2-fold axes go through two face centers and the other half go through 2 edge midpoints.
Answer: None of the above; unlike the uniform polyhedra, pyramids have no 2-fold axes of symmetry. The pentagonal pyramid has only a single 5-fold axis. This is called cyclic symmetry.