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Octahedron


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Polyhedron with 8 faces































































Regular octahedron

Octahedron.jpg
(Click here for rotating model)
Type
Platonic solid
Elements
F = 8, E = 12
V = 6 (χ = 2)
Faces by sides 8{3}
Conway notation O
aT
Schläfli symbols {3,4}
r{3,3} or {33}{displaystyle {begin{Bmatrix}3\3end{Bmatrix}}}
Face configuration V4.4.4
Wythoff symbol 4 | 2 3
Coxeter diagram
CDel node.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node 1.png
Symmetry
Oh, BC3, [4,3], (*432)
Rotation group
O, [4,3]+, (432)
References
U05, C17, W2
Properties
regular, convexdeltahedron
Dihedral angle 109.47122° = arccos(−1/3)

Octahedron vertfig.png
3.3.3.3
(Vertex figure)

Hexahedron.png
Cube
(dual polyhedron)

Octahedron flat.svg
Net

In geometry, an octahedron (plural: octahedra) is a polyhedron with eight faces, twelve edges, and six vertices. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.


A regular octahedron is the dual polyhedron of a cube. It is a rectified tetrahedron. It is a square bipyramid in any of three orthogonal orientations. It is also a triangular antiprism in any of four orientations.


An octahedron is the three-dimensional case of the more general concept of a cross polytope.


A regular octahedron is a 3-ball in the Manhattan (1) metric.




Contents






  • 1 Regular octahedron


    • 1.1 Dimensions


    • 1.2 Orthogonal projections


    • 1.3 Spherical tiling


    • 1.4 Cartesian coordinates


    • 1.5 Area and volume


    • 1.6 Geometric relations


    • 1.7 Uniform colorings and symmetry


    • 1.8 Nets


    • 1.9 Dual


    • 1.10 Faceting




  • 2 Irregular octahedra


    • 2.1 Other convex octahedra




  • 3 Octahedra in the physical world


    • 3.1 Octahedra in nature


    • 3.2 Octahedra in art and culture


    • 3.3 Tetrahedral Truss




  • 4 Related polyhedra


    • 4.1 Tetratetrahedron


    • 4.2 Trigonal antiprism


    • 4.3 Square bipyramid




  • 5 See also


  • 6 References


  • 7 External links





Regular octahedron[edit]



Dimensions[edit]


If the edge length of a regular octahedron is a, the radius of a circumscribed sphere (one that touches the octahedron at all vertices) is


ru=a22≈0.707⋅a{displaystyle r_{u}={frac {a}{2}}{sqrt {2}}approx 0.707cdot a}

and the radius of an inscribed sphere (tangent to each of the octahedron's faces) is


ri=a66≈0.408⋅a{displaystyle r_{i}={frac {a}{6}}{sqrt {6}}approx 0.408cdot a}

while the midradius, which touches the middle of each edge, is


rm=a2=0.5⋅a{displaystyle r_{m}={frac {a}{2}}=0.5cdot a}


Orthogonal projections[edit]


The octahedron has four special orthogonal projections, centered, on an edge, vertex, face, and normal to a face. The second and third correspond to the B2 and A2Coxeter planes.



























Orthogonal projections
Centered by
Edge
Face
Normal
Vertex
Face
Image

Cube t2 e.png

Cube t2 fb.png

3-cube t2 B2.svg

3-cube t2.svg
Projective
symmetry
[2]
[2]
[4]
[6]


Spherical tiling[edit]


The octahedron can also be represented as a spherical tiling, and projected onto the plane via a stereographic projection. This projection is conformal, preserving angles but not areas or lengths. Straight lines on the sphere are projected as circular arcs on the plane.












Uniform tiling 432-t2.png

Octahedron stereographic projection.svg

Orthographic projection

Stereographic projection


Cartesian coordinates[edit]


An octahedron with edge length 2 can be placed with its center at the origin and its vertices on the coordinate axes; the Cartesian coordinates of the vertices are then



( ±1, 0, 0 );

( 0, ±1, 0 );

( 0, 0, ±1 ).


In an xyz Cartesian coordinate system, the octahedron with center coordinates (a, b, c) and radius r is the set of all points (x, y, z) such that


|x−a|+|y−b|+|z−c|=r.{displaystyle left|x-aright|+left|y-bright|+left|z-cright|=r.}


Area and volume[edit]


The surface area A and the volume V of a regular octahedron of edge length a are:



A=23a2≈3.464a2{displaystyle A=2{sqrt {3}}a^{2}approx 3.464a^{2}}

V=132a3≈0.471a3{displaystyle V={frac {1}{3}}{sqrt {2}}a^{3}approx 0.471a^{3}}


Thus the volume is four times that of a regular tetrahedron with the same edge length, while the surface area is twice (because we have 8 rather than 4 triangles).


If an octahedron has been stretched so that it obeys the equation


|xxm|+|yym|+|zzm|=1,{displaystyle left|{frac {x}{x_{m}}}right|+left|{frac {y}{y_{m}}}right|+left|{frac {z}{z_{m}}}right|=1,}

the formulas for the surface area and volume expand to become



A=4xmymzm×1xm2+1ym2+1zm2,{displaystyle A=4,x_{m},y_{m},z_{m}times {sqrt {{frac {1}{x_{m}^{2}}}+{frac {1}{y_{m}^{2}}}+{frac {1}{z_{m}^{2}}}}},}

V=43xmymzm.{displaystyle V={frac {4}{3}},x_{m},y_{m},z_{m}.}


Additionally the inertia tensor of the stretched octahedron is


I=[110m(ym2+zm2)000110m(xm2+zm2)000110m(xm2−ym2)].{displaystyle I={begin{bmatrix}{frac {1}{10}}m(y_{m}^{2}+z_{m}^{2})&0&0\0&{frac {1}{10}}m(x_{m}^{2}+z_{m}^{2})&0\0&0&{frac {1}{10}}m(x_{m}^{2}-y_{m}^{2})end{bmatrix}}.}

These reduce to the equations for the regular octahedron when


xm=ym=zm=a22.{displaystyle x_{m}=y_{m}=z_{m}=a,{frac {sqrt {2}}{2}}.}


Geometric relations[edit]




The octahedron represents the central intersection of two tetrahedra


The interior of the compound of two dual tetrahedra is an octahedron, and this compound, called the stella octangula, is its first and only stellation. Correspondingly, a regular octahedron is the result of cutting off from a regular tetrahedron, four regular tetrahedra of half the linear size (i.e. rectifying the tetrahedron). The vertices of the octahedron lie at the midpoints of the edges of the tetrahedron, and in this sense it relates to the tetrahedron in the same way that the cuboctahedron and icosidodecahedron relate to the other Platonic solids. One can also divide the edges of an octahedron in the ratio of the golden mean to define the vertices of an icosahedron. This is done by first placing vectors along the octahedron's edges such that each face is bounded by a cycle, then similarly partitioning each edge into the golden mean along the direction of its vector. There are five octahedra that define any given icosahedron in this fashion, and together they define a regular compound.


Octahedra and tetrahedra can be alternated to form a vertex, edge, and face-uniform tessellation of space, called the octet truss by Buckminster Fuller. This is the only such tiling save the regular tessellation of cubes, and is one of the 28 convex uniform honeycombs. Another is a tessellation of octahedra and cuboctahedra.


The octahedron is unique among the Platonic solids in having an even number of faces meeting at each vertex. Consequently, it is the only member of that group to possess mirror planes that do not pass through any of the faces.


Using the standard nomenclature for Johnson solids, an octahedron would be called a square bipyramid. Truncation of two opposite vertices results in a square bifrustum.


The octahedron is 4-connected, meaning that it takes the removal of four vertices to disconnect the remaining vertices. It is one of only four 4-connected simplicial well-covered polyhedra, meaning that all of the maximal independent sets of its vertices have the same size. The other three polyhedra with this property are the pentagonal dipyramid, the snub disphenoid, and an irregular polyhedron with 12 vertices and 20 triangular faces.[1]



Uniform colorings and symmetry[edit]


There are 3 uniform colorings of the octahedron, named by the triangular face colors going around each vertex: 1212, 1112, 1111.


The octahedron's symmetry group is Oh, of order 48, the three dimensional hyperoctahedral group. This group's subgroups include D3d (order 12), the symmetry group of a triangular antiprism; D4h (order 16), the symmetry group of a square bipyramid; and Td (order 24), the symmetry group of a rectified tetrahedron. These symmetries can be emphasized by different colorings of the faces.



























































Name
Octahedron

Rectified tetrahedron
(Tetratetrahedron)
Triangular antiprism
Square bipyramid
Rhombic fusil
Image
(Face coloring)

Uniform polyhedron-43-t2.png
(1111)

Uniform polyhedron-33-t1.png
(1212)

Trigonal antiprism.png
(1112)

Square bipyramid.png
(1111)

Rhombic bipyramid.png
(1111)

Coxeter diagram

CDel node 1.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node.png

CDel node 1.pngCDel 3.pngCDel node.pngCDel 4.pngCDel node h0.png = CDel node 1.pngCDel split1.pngCDel nodes.png

CDel node h.pngCDel 2x.pngCDel node h.pngCDel 6.pngCDel node.png
CDel node h.pngCDel 2x.pngCDel node h.pngCDel 3.pngCDel node h.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 4.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node f1.png

Schläfli symbol
{3,4}
r{3,3}
s{2,6}
sr{2,3}
ft{2,4}
{ } + {4}
ftr{2,2}
{ } + { } + { }

Wythoff symbol
4 | 3 2
2 | 4 3
2 | 6 2
| 2 3 2


Symmetry
Oh, [4,3], (*432)
Td, [3,3], (*332)
D3d, [2+,6], (2*3)
D3, [2,3]+, (322)
D4h, [2,4], (*422)
D2h, [2,2], (*222)

Order
48
24
12
6
16
8


Nets[edit]


It has eleven arrangements of nets.



Dual[edit]


The octahedron is the dual polyhedron to the cube.


Dual Cube-Octahedron.svg


Faceting[edit]


The uniform tetrahemihexahedron is a tetrahedral symmetry faceting of the regular octahedron, sharing edge and vertex arrangement. It has four of the triangular faces, and 3 central squares.






Uniform polyhedron-33-t1.png
Octahedron

Tetrahemihexahedron.png
Tetrahemihexahedron


Irregular octahedra[edit]


The following polyhedra are combinatorially equivalent to the regular polyhedron. They all have six vertices, eight triangular faces, and twelve edges that correspond one-for-one with the features of a regular octahedron.




  • Triangular antiprisms: Two faces are equilateral, lie on parallel planes, and have a common axis of symmetry. The other six triangles are isosceles.

  • Tetragonal bipyramids, in which at least one of the equatorial quadrilaterals lies on a plane. The regular octahedron is a special case in which all three quadrilaterals are planar squares.


  • Schönhardt polyhedron, a non-convex polyhedron that cannot be partitioned into tetrahedra without introducing new vertices.


  • Bricard octahedron, a non-convex self-crossing flexible polyhedron



Other convex octahedra[edit]


More generally, an octahedron can be any polyhedron with eight faces. The regular octahedron has 6 vertices and 12 edges, the minimum for an octahedron; irregular octahedra may have as many as 12 vertices and 18 edges.[2]
There are 257 topologically distinct convex octahedra, excluding mirror images. More specifically there are 2, 11, 42, 74, 76, 38, 14 for octahedra with 6 to 12 vertices respectively.[3][4] (Two polyhedra are "topologically distinct" if they have intrinsically different arrangements of faces and vertices, such that it is impossible to distort one into the other simply by changing the lengths of edges or the angles between edges or faces.)


Some better known irregular octahedra include the following:




  • Hexagonal prism: Two faces are parallel regular hexagons; six squares link corresponding pairs of hexagon edges.

  • Heptagonal pyramid: One face is a heptagon (usually regular), and the remaining seven faces are triangles (usually isosceles). It is not possible for all triangular faces to be equilateral.


  • Truncated tetrahedron: The four faces from the tetrahedron are truncated to become regular hexagons, and there are four more equilateral triangle faces where each tetrahedron vertex was truncated.


  • Tetragonal trapezohedron: The eight faces are congruent kites.



Octahedra in the physical world[edit]



Octahedra in nature[edit]





Fluorite octahedron.



  • Natural crystals of diamond, alum or fluorite are commonly octahedral, as the space-filling tetrahedral-octahedral honeycomb.

  • The plates of kamacite alloy in octahedrite meteorites are arranged paralleling the eight faces of an octahedron.

  • Many metal ions coordinate six ligands in an octahedral or distorted octahedral configuration.


  • Widmanstätten patterns in nickel-iron crystals



Octahedra in art and culture[edit]




Two identically formed rubik's snakes can approximate an octahedron.



  • Especially in roleplaying games, this solid is known as a "d8", one of the more common polyhedral dice.

  • In the film Tron (1982), the character Bit took this shape as the "Yes" state.

  • If each edge of an octahedron is replaced by a one-ohm resistor, the resistance between opposite vertices is 1/2 ohm, and that between adjacent vertices 5/12 ohm.[5]

  • Six musical notes can be arranged on the vertices of an octahedron in such a way that each edge represents a consonant dyad and each face represents a consonant triad; see hexany.



Tetrahedral Truss[edit]


A framework of repeating tetrahedrons and octahedrons was invented by Buckminster Fuller in the 1950s, known as a space frame, commonly regarded as the strongest structure for resisting cantilever stresses.



Related polyhedra[edit]


A regular octahedron can be augmented into a tetrahedron by adding 4 tetrahedra on alternated faces. Adding tetrahedra to all 8 faces creates the stellated octahedron.












Triangulated tetrahedron.png

Compound of two tetrahedra.png

tetrahedron

stellated octahedron

The octahedron is one of a family of uniform polyhedra related to the cube.




















































































































It is also one of the simplest examples of a hypersimplex, a polytope formed by certain intersections of a hypercube with a hyperplane.


The octahedron is topologically related as a part of sequence of regular polyhedra with Schläfli symbols {3,n}, continuing into the hyperbolic plane.









































Tetratetrahedron[edit]


The regular octahedron can also be considered a rectified tetrahedron – and can be called a tetratetrahedron. This can be shown by a 2-color face model. With this coloring, the octahedron has tetrahedral symmetry.


Compare this truncation sequence between a tetrahedron and its dual:




























































The above shapes may also be realized as slices orthogonal to the long diagonal of a tesseract. If this diagonal is oriented vertically with a height of 1, then the first five slices above occur at heights r, 3/8, 1/2, 5/8, and s, where r is any number in the range 0 < r1/4, and s is any number in the range 3/4s < 1.


The octahedron as a tetratetrahedron exists in a sequence of symmetries of quasiregular polyhedra and tilings with vertex configurations (3.n)2, progressing from tilings of the sphere to the Euclidean plane and into the hyperbolic plane. With orbifold notation symmetry of *n32 all of these tilings are Wythoff constructions within a fundamental domain of symmetry, with generator points at the right angle corner of the domain.[6][7]









































Trigonal antiprism[edit]


As a trigonal antiprism, the octahedron is related to the hexagonal dihedral symmetry family.



















































































































Square bipyramid[edit]






















































Family of bipyramids
Polyhedron


Triangular bipyramid.png

Square bipyramid.png

Pentagonale bipiramide.png

Hexagonale bipiramide.png

Heptagonal bipyramid.png

Octagonal bipyramid.png

Enneagonal bipyramid.png

Decagonal bipyramid.png

Coxeter

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 2x.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 3.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 4.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 5.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 6.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 7.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 8.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 9.pngCDel node.png

CDel node f1.pngCDel 2x.pngCDel node f1.pngCDel 10.pngCDel node.png
Tiling

Spherical digonal bipyramid.svg

Spherical trigonal bipyramid.png

Spherical square bipyramid.svg

Spherical pentagonal bipyramid.png

Spherical hexagonal bipyramid.png

Spherical heptagonal bipyramid.png

Spherical octagonal bipyramid.png

Spherical enneagonal bipyramid.png

Spherical decagonal bipyramid.png

Config.
V2.4.4

V3.4.4

V4.4.4

V5.4.4

V6.4.4

V7.4.4

V8.4.4
V9.4.4

V10.4.4


See also[edit]



  • Octahedral number

  • Centered octahedral number

  • Spinning octahedron

  • Stella octangula

  • Triakis octahedron

  • Hexakis octahedron

  • Truncated octahedron

  • Octahedral molecular geometry

  • Octahedral symmetry

  • Octahedral graph



References[edit]





  1. ^ Finbow, Arthur S.; Hartnell, Bert L.; Nowakowski, Richard J.; Plummer, Michael D. (2010). "On well-covered triangulations. III". Discrete Applied Mathematics. 158 (8): 894–912. doi:10.1016/j.dam.2009.08.002. MR 2602814..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}


  2. ^ [1]


  3. ^ Counting polyhedra


  4. ^ "Archived copy". Archived from the original on 17 November 2014. Retrieved 14 August 2016.CS1 maint: Archived copy as title (link)


  5. ^ Klein, Douglas J. (2002). "Resistance-Distance Sum Rules" (PDF). Croatica Chemica Acta. 75 (2): 633–649. Retrieved 2006-09-30.


  6. ^ Coxeter Regular Polytopes, Third edition, (1973), Dover edition,
    ISBN 0-486-61480-8 (Chapter V: The Kaleidoscope, Section: 5.7 Wythoff's construction)



  7. ^ Two Dimensional symmetry Mutations by Daniel Huson




External links[edit]




  • Wikisource "Octahedron". Encyclopædia Britannica. 19 (11th ed.). 1911.

  • Weisstein, Eric W. "Octahedron". MathWorld.


  • Klitzing, Richard. "3D convex uniform polyhedra x3o4o - oct".

  • Editable printable net of an octahedron with interactive 3D view

  • Paper model of the octahedron

  • K.J.M. MacLean, A Geometric Analysis of the Five Platonic Solids and Other Semi-Regular Polyhedra

  • The Uniform Polyhedra


  • Virtual Reality Polyhedra The Encyclopedia of Polyhedra

    • Conway Notation for Polyhedra Try: dP4



































































































Fundamental convex regular and uniform polytopes in dimensions 2–10


Family

An

Bn

I2(p) / Dn

E6 / E7 / E8 / F4 / G2

Hn

Regular polygon

Triangle

Square

p-gon

Hexagon

Pentagon

Uniform polyhedron

Tetrahedron

Octahedron • Cube

Demicube


Dodecahedron • Icosahedron

Uniform 4-polytope

5-cell

16-cell • Tesseract

Demitesseract

24-cell

120-cell • 600-cell

Uniform 5-polytope

5-simplex

5-orthoplex • 5-cube

5-demicube



Uniform 6-polytope

6-simplex

6-orthoplex • 6-cube

6-demicube

122 • 221


Uniform 7-polytope

7-simplex

7-orthoplex • 7-cube

7-demicube

132 • 231 • 321


Uniform 8-polytope

8-simplex

8-orthoplex • 8-cube

8-demicube

142 • 241 • 421


Uniform 9-polytope

9-simplex

9-orthoplex • 9-cube

9-demicube



Uniform 10-polytope

10-simplex

10-orthoplex • 10-cube

10-demicube


Uniform n-polytope

n-simplex

n-orthoplex • n-cube

n-demicube

1k2 • 2k1 • k21

n-pentagonal polytope
Topics: Polytope families • Regular polytope • List of regular polytopes and compounds












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