In geometry, a prism is a polyhedron comprising an polygon base, a second base which is a translated copy (rigidly moved without rotation) of the first, and other faces, necessarily all , joining corresponding sides of the two bases. All cross-sections parallel to the bases are translations of the bases. Prisms are named after their bases, e.g. a prism with a pentagonal base is called a pentagonal prism. Prisms are a subclass of .
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, this definition has been criticized for not being specific enough in regard to the nature of the bases (a cause of some confusion amongst generations of later geometry writers).
Oblique vs right
An
oblique prism is a prism in which the joining edges and faces are
not perpendicular to the base faces.
Example: a parallelepiped is an oblique prism whose base is a parallelogram, or equivalently a polyhedron with six parallelogram faces.
A right prism is a prism in which the joining edges and faces are perpendicular to the base faces.
The Dual polyhedron of a right -prism is a right -bipyramid.
A right prism (with rectangular sides) with Regular polygon bases has Schläfli symbol It approaches a cylinder as approaches infinity.
Special cases
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A right rectangular prism (with a rectangular base) is also called a cuboid, or informally a rectangular box. A right rectangular prism has Schläfli symbol
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A right square prism (with a square base) is also called a square cuboid, or informally a square box.
Note: some texts may apply the term rectangular prism or square prism to both a right rectangular-based prism and a right square-based prism.
Types
Regular prism
A
regular prism is a prism with
Regular polygon bases.
Uniform prism
A
uniform prism or
semiregular prism is a right prism with regular bases and all edges of the same length.
Thus all the side faces of a uniform prism are .
Thus all the faces of a uniform prism are regular polygons. Also, such prisms are Isogonal figure; thus they are uniform polyhedra. They form one of the two infinite series of semiregular polyhedra, the other series being formed by the .
A uniform -gonal prism has Schläfli symbol
Properties
Volume
The
volume of a prism is the product of the
area of the base by the height, i.e. the distance between the two base faces (in the case of a non-right prism, note that this means the perpendicular distance).
The volume is therefore:
where is the base area and is the height.
The volume of a prism whose base is an -sided regular polygon with side length is therefore:
Surface area
The surface
area of a right prism is:
where is the area of the base, the height, and the base
perimeter.
The surface area of a right prism whose base is a regular -sided polygon with side length , and with height , is therefore:
Symmetry
The
symmetry group of a right -sided prism with regular base is of order , except in the case of a cube, which has the larger symmetry group of order 48, which has three versions of as
. The rotation group is of order , except in the case of a cube, which has the larger symmetry group of order 24, which has three versions of as subgroups.
The symmetry group contains Point reflection iff is even.
The Hosohedron and Dihedron also possess dihedral symmetry, and an -gonal prism can be constructed via the geometrical truncation of an -gonal hosohedron, as well as through the cantellation or expansion of an -gonal dihedron.
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Similar polytopes
Truncated prism
A
truncated prism is formed when prism is sliced by a plane that is not parallel to its bases. A truncated prism's bases are not congruent, and its sides are not parallelograms.
Twisted prism
A
twisted prism is a nonconvex polyhedron constructed from a uniform -prism with each side face bisected on the square diagonal, by twisting the top, usually (but not necessarily) by radians ( degrees). If the bisectors are slanted to the left, then twisting the top base in the right direction (looking at the top of the prism) by a small angle gives nonconvex polyhedron and twisting it in the left direction, a convex polyhedron (see twisted square prism on the image). If the bisectors are slanted to the right, then twisting the top base in the left direction gives nonconvex polyhedron, in the right direction, convex one (see twisted dodecagonal prism).
A twisted prism cannot be dissected into tetrahedra without adding new vertices. The simplest twisted prism has triangle bases and is called a Schönhardt polyhedron.
An -gonal twisted prism is topologically identical to the -gonal uniform antiprism, but has half the symmetry group: , order . It can be seen as a nonconvex antiprism, with tetrahedra removed between pairs of triangles. Any twisted -gonal prism is an antiprism, so the twisted square prism and twisted dodecagonal prism shown on the image are both antiprisms.
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| Schönhardt polyhedron | Twisted square prism | Square antiprism | Twisted dodecagonal prism |
Frustum
A
frustum is a similar construction to a prism, with
trapezoid lateral faces and differently 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 rectangles and 2 faces. It is topologically identical to a -gonal prism.
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Crossed prism
A
crossed prism is a nonconvex polyhedron constructed from a prism, where the vertices of one base are
Point reflection of this base (or rotated by 180°). This transforms the side rectangular faces into crossed rectangles. For a regular polygon base, the appearance is an -gonal
hour glass. All oblique edges pass through a single body center. Note: no vertex is at this body centre. A crossed prism is topologically identical to an -gonal prism.
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Toroidal prism
A
toroidal prism is a nonconvex polyhedron like a
crossed prism, but without bottom and top base faces, and with simple rectangular side faces closing 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 configuration ): a band of squares, each attached to a
Rectangle. An -gonal toroidal prism has vertices, faces: squares and crossed rectangles, and edges. It is topologically self-dual.
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Prismatic polytope
A
prismatic polytope is a higher-dimensional generalization of a prism. An -dimensional prismatic polytope is constructed from two ()-dimensional polytopes, translated into the next dimension.
The prismatic -polytope elements are doubled from the ()-polytope elements and then creating new elements from the next lower element.
Take an -polytope with -face elements (). Its ()-polytope prism will have -face elements. (With , .)
By dimension:
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Take a polygon with vertices, edges. Its prism has vertices, edges, and faces.
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Take a polyhedron with vertices, edges, and faces. Its prism has vertices, edges, faces, and cells.
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Take a polychoron with vertices, edges, faces, and cells. Its prism has vertices, edges, faces, cells, and hypercells.
Uniform prismatic polytope
A regular -polytope represented by Schläfli symbol can form a uniform prismatic ()-polytope represented by a Cartesian product of two Schläfli symbols:
By dimension:
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A 0-polytopic prism is a line segment, represented by an empty Schläfli symbol
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:
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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:
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:Example: , Square, two parallel line segments, connected by two line segment sides.
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A prism is a 3-dimensional prism made from two translated polygons connected by rectangles. A regular polygon can construct a uniform -gonal prism represented by the product If , with square sides symmetry it becomes a cube:
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:Example: , Pentagonal prism, two parallel connected by 5 rectangular sides.
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A Polyhedron 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 and the sides are cubes, it becomes a tesseract:
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:Example: , Dodecahedral prism, two parallel dodecahedra connected by 12 pentagonal prism sides.
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Higher order prismatic polytopes also exist as cartesian products of any two or more polytopes. The dimension of a product polytope is the sum of the dimensions of its elements. The first examples of these exist in 4-dimensional space; they are called
as the product of two polygons in 4-dimensions.
Regular duoprisms are represented as with vertices, edges, square faces, -gon faces, -gon faces, and bounded by -gonal prisms and -gonal prisms.
For example, a 4-4 duoprism is a lower symmetry form of a tesseract, as is a cubic prism. (4-4 duoprism prism), (cube-4 duoprism) and (tesseractic prism) are lower symmetry forms of a 5-cube.
See also
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Chapter 2: Archimedean polyhedra, prisma and antiprisms
External links
