Prism (geometry)
In geometry, a prism is a polyhedron comprising an polygon base, a second base which is a translated copy of the first, and other faces, necessarily all parallelograms, 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 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, this definition has been criticized for not being specific enough in regard to the nature of the bases.
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. This applies if and only if all the joining faces are rectangular.
The dual of a right -prism is a right -bipyramid.
A right prism with regular -gon bases has Schläfli symbol It approaches a cylinder as approaches infinity.
Special cases
- A right rectangular prism is also called a cuboid, or informally a rectangular box. A right rectangular prism has Schläfli symbol
- A right square prism is also called a square cuboid, or informally a square box.
Types
Regular prism
A regular prism is a prism with regular 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 squares.
Thus all the faces of a uniform prism are regular polygons. Also, such prisms are isogonal; thus they are uniform polyhedra. They form one of the two infinite series of semiregular polyhedra, the other series being formed by the antiprisms.
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.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 subgroups. 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 inversion iff is even.
The hosohedra and dihedra 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.
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 by radians. If the bisectors are slanted to the left, then twisting the top base in the right direction by a small angle gives nonconvex polyhedron and twisting it in the left direction, a convex polyhedron. 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.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.
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.Crossed prism
A crossed prism is a nonconvex polyhedron constructed from a prism, where the vertices of one base are inverted around the center of this base. 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.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 : a band of squares, each attached to a crossed rectangle. An -gonal toroidal prism has vertices, faces: squares and crossed rectangles, and edges. It is topologically self-dual.| , order 16 | , order 24 |
| ,, | ,, |
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.
By dimension:
- Take a polygon with vertices, edges. Its prism has vertices, edges, and faces.
- Take a polyhedron with vertices, edges, and faces. Its prism has vertices, edges, faces, and cells.
- 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:
- A 0-polytopic prism is a line segment, represented by an empty Schläfli symbol
- :[Image:Complete graph K2.svg|class=skin-invert|60px]
- 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: [Image:Square diagonals.svg|class=skin-invert|60px], 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 -gonal prism represented by the product If, with square sides symmetry it becomes a cube:
- :Example: 60px, 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 and the sides are cubes, it becomes a tesseract:
- :Example: 50px, Dodecahedral prism, two parallel dodecahedra connected by 12 pentagonal prism sides.
- ...
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., and are lower symmetry forms of a 5-cube.