Prism (geometry)
In geometry, a prism is a polyhedron comprising an n-sided polygonal base, a second base which is a translated copy of the first, and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the bases are translations of the bases. Prisms are named for their bases, so a prism with a pentagonal base is called a pentagonal prism. The prisms are a subclass of the prismatoids.
Like many basic geometric terms, the word prism was first used in Euclid's Elements. Euclid defined the term in Book XI as “a solid figure contained by two opposite, equal and parallel planes, while the rest are parallelograms”, however in nine subsequent propositions using the term he included examples of triangular-based prisms. This inconsistency caused confusion amongst later geometricians.
General, right and uniform prisms
A right prism is a prism in which the joining edges and faces are perpendicular to the base faces. This applies if the joining faces are rectangular. If the joining edges and faces are not perpendicular to the base faces, it is called an oblique prism.For example a parallelepiped is an oblique prism of which the base is a parallelogram, or equivalently a polyhedron with six faces which are all parallelograms.
A truncated prism is a prism with nonparallel top and bottom faces.
Some texts may apply the term rectangular prism or square prism to both a right rectangular-sided prism and a right square-sided prism. A right p-gonal prism with rectangular sides has a Schläfli symbol ×.
A right rectangular prism is also called a cuboid, or informally a rectangular box. A right square prism is simply a square box, and may also be called a square cuboid. A right rectangular prism has Schläfli symbol ××.
An n-prism, having regular polygon ends and rectangular sides, approaches a cylindrical solid as n approaches infinity.
The term uniform prism or semiregular prism can be used for a right prism with square sides, since such prisms are in the set of uniform polyhedra. A uniform p-gonal prism has a Schläfli symbol t. Right prisms with regular bases and equal edge lengths form one of the two infinite series of semiregular polyhedra, the other series being the antiprisms.
The dual of a right prism is a bipyramid.
Volume
The volume of a prism is the product of the area of the base and the distance between the two base faces, or the height.The volume is therefore:
where B is the base area and h is the height. The volume of a prism whose base is an n-sided regular polygon with side length s is therefore:
Surface area
The surface area of a right prism is:where B is the area of the base, h the height, and P the base perimeter.
The surface area of a right prism whose base is a regular n-sided polygon with side length s and height h is therefore:
[Schlegel diagram]s
Symmetry
The symmetry group of a right n-sided prism with regular base is Dnh of order 4n, except in the case of a cube, which has the larger symmetry group Oh of order 48, which has three versions of D4h as subgroups. The rotation group is Dn of order 2n, except in the case of a cube, which has the larger symmetry group O of order 24, which has three versions of D4 as subgroups.The symmetry group Dnh contains inversion iff n is even.
The hosohedra and dihedra also possess dihedral symmetry, and a n-gonal prism can be constructed via the geometrical truncation of a n-gonal hosohedron, as well as through the cantellation or expansion of a n-gonal dihedron.
Prismatic polytope
A prismatic polytope is a higher-dimensional generalization of a prism. An n-dimensional prismatic polytope is constructed from two -dimensional polytopes, translated into the next dimension.The prismatic n-polytope elements are doubled from the -polytope elements and then creating new elements from the next lower element.
Take an n-polytope with fi i-face elements. Its -polytope prism will have i-face elements.
By dimension:
- Take a polygon with n vertices, n edges. Its prism has 2n vertices, 3n edges, and faces.
- Take a polyhedron with v vertices, e edges, and f faces. Its prism has 2v vertices, edges, faces, and cells.
- Take a polychoron with v vertices, e edges, f faces and c cells. Its prism has 2v vertices, edges, faces, and cells, and hypercells.
Uniform prismatic polytope
By dimension:
- A 0-polytopic prism is a line segment, represented by an empty Schläfli symbol.
- *
- A 1-polytopic prism is a rectangle, made from 2 translated line segments. It is represented as the product Schläfli symbol ×. If it is square, symmetry can be reduced:
- *Example: Square, ×, two parallel line segments, connected by two line segment sides.
- A polygonal prism is a 3-dimensional prism made from two translated polygons connected by rectangles. A regular polygon can construct a uniform n-gonal prism represented by the product ×. If, with square sides symmetry it becomes a cube:
- *Example: Pentagonal prism, ×, two parallel pentagons connected by 5 rectangular sides.
- A polyhedral prism is a 4-dimensional prism made from two translated polyhedra connected by 3-dimensional prism cells. A regular polyhedron can construct the uniform polychoric prism, represented by the product ×. If the polyhedron is a cube, and the sides are cubes, it becomes a tesseract: × =
- *Example: Dodecahedral prism, ×, two parallel dodecahedra connected by 12 pentagonal prism sides.
- ...
Twisted prism
A twisted prism is a nonconvex prism polyhedron constructed by a uniform q-prism with the side faces bisected on the square diagonal, and twisting the top, usually by radians in the same direction, causing side triangles to be concave.A twisted prism cannot be dissected into tetrahedra without adding new vertices. The smallest case, triangular form, is called a Schönhardt polyhedron.
A twisted prism is topologically identical to the antiprism, but has half the symmetry: Dn, +, order 2n. It can be seen as a convex antiprism, with tetrahedra removed between pairs of triangles.
Frustum
A frustum is topologically identical to a prism, with trapezoid lateral faces and different sized top and bottom polygons.Star prism
A star prism is a nonconvex polyhedron constructed by two identical star polygon faces on the top and bottom, being parallel and offset by a distance and connected by rectangular faces. A uniform star prism will have Schläfli symbol ×, with p rectangle and 2 faces. It is topologically identical to a p-gonal prism.Crossed prism
A crossed prism is a nonconvex polyhedron constructed from a prism, where the base vertices are inverted around the center. This transforms the side rectangular faces into crossed rectangles. For a regular polygon base, the appearance is an p-gonal hour glass, with all vertical edges passing through a single center, but no vertex is there. It is topologically identical to a p-gonal prism.Toroidal prisms
A toroidal prism is a nonconvex polyhedron is like a crossed prism except instead of having base and top polygons, simple rectangular side faces are added to close the polyhedron. This can only be done for even-sided base polygons. These are topological tori, with Euler characteristic of zero. The topological polyhedral net can be cut from two rows of a square tiling, with vertex figure 4.4.4.4. A n-gonal toroidal prism has 2n vertices and faces, and 4n edges and is topologically self-dual.| D4h, order 16 | D6h, order 24 |
| v=8, e=16, f=8 | v=12, e=24, f=12 |