Introduction

Mathematicians love to classify and count things. We now apply this idea to polyhedra, making some attractive and interesting models along the way.

A regular convex polygon is a closed 2-dimensional figure whose edges have the same length and meet at the same angle at every vertex. For example, a equilateral triangle or a square is a regular convex polygon. There are an infinite number of regular convex polygons.

If we take the same idea and scale it up to three dimensions, we get regular convex polyhedra. That is, a regular convex polyhedron is a closed 3-dimensional object made up of congruent regular convex polygons so that the same number of faces meet at each vertex. However, instead of having an infinite number of regular convex polyhedra, there are only 5, the Platonic solids.

The Platonic solids were known to the Greeks. Plato believed that the cube, tetrahedron, octahedron, and the icosahedron represented the shapes of the fundamental atoms of earth, fire, air, and water. To the fifth regular convex polyhedron, the dodecahedron, he associated the whole universe.

cube

tetrahedron

octahedron

dodecahedron

icosahedron

Exercise

Use the polydrons to make models of the 5 Platonic solids, and come up with a convincing argument that there are only 5.

Questions

Exercises

Perhaps you just realized that the truncated icosahedron is formed by dividing the edges of an icosahedron into thirds and then slicing the corners off of it at these points. We can perform this procedure to each of the Platonic solids and get 4 more Archimedean solids.

1. Think about what each of these polyhedron look like. What are the faces and how are they arranged at each vertex? Use the polydrons to make models of some of these polyhedra. What do you think they are called?

   Pictures of the polyhedra can be found at http://home.teleport.com/~tpgettys/trplato.shtml

2. In the above procedure, we sliced the corners off of the solids a third of the way in from each vertex. What if we divide the edges in half and then truncate? How many more Archimedean solids do we get in this way?

   Answer

Other Links*

There are 6 more Archimedean solids. One of many sites for reading about and viewing all 13 Archimedean solids is http://home.teleport.com/~tpgettys/archimed.shtml

Now the enquiring mind may want to know whether the prisms, anti-prisms, and Archimedean solids are all of the semi-regular convex polyhedra. There is exactly one more, the pseudorhombicuboctahedron or elongated square gyrobicupola, which is obtained by rotating the bottom third of the small rhombicuboctahedron. Each vertex has a triangle and then three squares. However, it is not considered an Archimedean solid because though each vertex looks the same locally, the polyhedron does not have as many symmetries as the Archimedean solids.

For more information and a picture of the pseudorhombicuboctahedron visit http://euch3i.chem.emory.edu/proposal/www.li.net/~george/virtual-polyhedra/pseudo-rhombicuboctahedra.html

There is a lot of information on-line about polyhedra and much of it is even correct. With the right software, you can even view virtual 3-dimensional models and rotate them with your mouse. You might want to start with the following sites:

• http://www.georgehart.com/virtual-polyhedra/vp.html
• http://home.teleport.com/~tpgettys/poly.shtml
• http://www.mathconsult.ch/showroom/unipoly/index.html

To build your own paper models of the Platonic solid earth maps, print out the patterns on card stock from http://ccins.camosun.bc.ca/~jbritton/polymap/jbpolymap.htm

* These links are for informational purposes only and are from sources outside MEC and Cornell University