Prism (geometry)

For other uses, see Prism (disambiguation).
Set of uniform prisms

(A hexagonal prism is shown)
Typeuniform polyhedron
Faces2+n total:
2 {n}
n {4}
Edges3n
Vertices2n
Schläfli symbol{n}×{} or t{2, n}
Coxeter diagram
Vertex configuration4.4.n
Symmetry groupDnh, [n,2], (*n22), order 4n
Rotation groupDn, [n,2]+, (n22), order 2n
Dual polyhedronbipyramids
Propertiesconvex, semi-regular vertex-transitive

n-gonal prism net (n = 9 here)

In geometry, a prism is a polyhedron with an n-sided polygonal base, another congruent parallel base (with the same rotational orientation), and n other faces (necessarily all parallelograms) joining corresponding sides of the two bases. All cross-sections parallel to the base faces are congruent to the bases. Prisms are named for their base, so a prism with a pentagonal base is called a pentagonal prism. The prisms are a subclass of the prismatoids.

General, right and uniform prisms

A right prism is a prism in which the joining edges and faces are perpendicular to the base faces.[1] 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 triangular prism with its top face truncated at an oblique angle

A truncated prism is a prism with nonparallel top and bottom faces.[2]


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 { } × {p}.

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{2,p}. 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 (in the case of a non-right prism, note that this means the perpendicular distance).

The volume is therefore:

V = B \cdot h

where B is the base area and h is the height. The volume of a prism whose base is a regular n-sided polygon with side length s is therefore:

V = \frac{n}{4}hs^2 \cot\frac{\pi}{n}.

Surface area

The surface area of a right prism is 2 · B + P · h, 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:

A = \frac{n}{2} s^2 \cot{\frac{\pi}{n}} + n s h.

Schlegel diagrams


P3
P4
P5
P6
P7
P8

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.

Prismatic polytope

A prismatic polytope is a higher-dimensional generalization of a prism. An n-dimensional prismatic polytope is constructed from two (n − 1)-dimensional polytopes, translated into the next dimension.

The prismatic n-polytope elements are doubled from the (n − 1)-polytope elements and then creating new elements from the next lower element.

Take an n-polytope with fi i-face elements (i = 0, ..., n). Its (n + 1)-polytope prism will have 2fi + fi−1 i-face elements. (With f−1 = 0, fn = 1.)

By dimension:

Uniform prismatic polytope

A regular n-polytope represented by Schläfli symbol {p, q, ..., t} can form a uniform prismatic (n + 1)-polytope represented by a Cartesian product of two Schläfli symbols: {p, q, ..., t}×{}.

By dimension:

Higher order prismatic polytopes also exist as cartesian products of any two polytopes. The dimension of a polytope is the product of the dimensions of the elements. The first example of these exist in 4-dimensional space are called duoprisms as the product of two polygons. Regular duoprisms are represented as {p}×{q}.

Family of uniform prisms
Polyhedron
Coxeter
Tiling
Config. 3.4.4 4.4.4 5.4.4 6.4.4 7.4.4 8.4.4 9.4.4 10.4.4 11.4.4 12.4.4

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 180/q degrees in the same direction, causing side triangles to be concave.[3][4]

A twisted prism cannot be triangulated 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, [n,2]+, order 2n. It can be seen as a convex antiprism, with tetrahedra removed between pairs of triangles.

3-gonal 4-gonal 12-gonal

Schönhardt polyhedron
Twisted square prism
Square antiprism
Twisted dodecagonal antiprism

See also

References

  1. ↑ William F. Kern, James R Bland,Solid Mensuration with proofs, 1938, p.28
  2. ↑ William F. Kern, James R Bland,Solid Mensuration with proofs, 1938, p.81
  3. ↑ The facts on file: Geometry handbook, Catherine A. Gorini, 2003, ISBN 0-8160-4875-4, p.172
  4. ↑

External links

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