Planar Region

In Euclidean geometry, a plane is a flat two-dimensional surface that extends indefinitely.
Euclidean planes often arise as subspaces of three-dimensional space $\mathbb^3$.
A prototypical example is one of a room's walls, infinitely extended and assumed infinitesimally thin.
While a pair of real numbers $\mathbb^2$ suffices to describe points on a plane, the relationship with out-of-plane points requires special consideration for their embedding in the ambient space $\mathbb^3$.

Derived concepts

A or (or simply "plane", in lay use) is a planar surface region; it is analogous to a line segment.
A ''bivector'' is an oriented plane segment, analogous to directed line segments.
A ''face'' is a plane segment bounding a solid object.
A ''slab'' is a region bounded by two parallel planes.
A ''parallelepiped'' is a region bounded by three pairs of parallel planes.

Background

Euclid set forth the first great landmark of mathematical thought, an axiomatic treatment of geometry. He selected a small core of undefined terms (called ''common notions'') and postulates (or axioms) which he then used to prove various geometrical statements. Although the plane in its modern sense is not directly given a definition anywhere in the '' Elements'', it may be thought of as part of the common notions. Euclid never used numbers to measure length, angle, or area. The Euclidean plane equipped with a chosen Cartesian coordinate system is called a ''Cartesian plane''; a non-Cartesian Euclidean plane equipped with a polar coordinate system would be called a ''polar plane''.

A plane is a ruled surface.

Euclidean plane

Representation

This section is solely concerned with planes embedded in three dimensions: specifically, in .

Determination by contained points and lines

In a Euclidean space of any number of dimensions, a plane is uniquely determined by any of the following:
* Three non-collinear points (points not on a single line).
* A line and a point not on that line.
* Two distinct but intersecting lines.
* Two distinct but parallel lines.

Properties

The following statements hold in three-dimensional Euclidean space but not in higher dimensions, though they have higher-dimensional analogues:
* Two distinct planes are either parallel or they intersect in a line.
* A line is either parallel to a plane, intersects it at a single point, or is contained in the plane.
* Two distinct lines perpendicular to the same plane must be parallel to each other.
* Two distinct planes perpendicular to the same line must be parallel to each other.

Point–normal form and general form of the equation of a plane

In a manner analogous to the way lines in a two-dimensional space are described using a point-slope form for their equations, planes in a three dimensional space have a natural description using a point in the plane and a vector orthogonal to it (the normal vector) to indicate its "inclination".

Specifically, let be the position vector of some point , and let be a nonzero vector. The plane determined by the point and the vector consists of those points , with position vector , such that the vector drawn from to is perpendicular to . Recalling that two vectors are perpendicular if and only if their dot product is zero, it follows that the desired plane can be described as the set of all points such that
$\boldsymbol \cdot (\boldsymbol-\boldsymbol_0)=0.$
The dot here means a dot (scalar) product.

Expanded this becomes
$a (x-x_0) + b(y-y_0) + c(z-z_0) = 0,$
which is the ''point–normal'' form of the equation of a plane. This is just a linear equation
$ax + by + cz + d = 0,$
where
$d = -(ax_0 + by_0 + cz_0),$
which is the expanded form of $- \boldsymbol \cdot \boldsymbol_0.$

In mathematics it is a common convention to express the normal as a unit vector, but the above argument holds for a normal vector of any non-zero length.

Conversely, it is easily shown that if , , , and are constants and , , and are not all zero, then the graph of the equation
$ax + by + cz + d = 0,$
is a plane having the vector as a normal. This familiar equation for a plane is called the ''general form'' of the equation of the plane or just the ''plane equation''.

Thus for example a regression equation of the form (with ) establishes a best-fit plane in three-dimensional space when there are two explanatory variables.

Describing a plane with a point and two vectors lying on it

Alternatively, a plane may be described parametrically as the set of all points of the form
$\boldsymbol = \boldsymbol_0 + s \boldsymbol + t \boldsymbol,$

where and range over all real numbers, and are given linearly independent vectors defining the plane, and is the vector representing the position of an arbitrary (but fixed) point on the plane. The vectors and can be visualized as vectors starting at and pointing in different directions along the plane. The vectors and can be perpendicular, but cannot be parallel.

Describing a plane through three points

Let , , and be non-collinear points.

Method 1

The plane passing through , , and can be described as the set of all points (''x'',''y'',''z'') that satisfy the following determinant equations:
$\begin x - x_1 & y - y_1 & z - z_1 \\ x_2 - x_1 & y_2 - y_1 & z_2 - z_1 \\ x_3 - x_1 & y_3 - y_1 & z_3 - z_1 \end = \begin x - x_1 & y - y_1 & z - z_1 \\ x - x_2 & y - y_2 & z - z_2 \\ x - x_3 & y - y_3 & z - z_3 \end = 0.$

Method 2

To describe the plane by an equation of the form $ax + by + cz + d = 0$, solve the following system of equations:
$ax_1 + by_1 + cz_1 + d = 0$
$ax_2 + by_2 + cz_2 + d = 0$
$ax_3 + by_3 + cz_3 + d = 0.$

This system can be solved using Cramer's rule and basic matrix manipulations. Let
$D = \begin x_1 & y_1 & z_1 \\ x_2 & y_2 & z_2 \\ x_3 & y_3 & z_3 \end.$

If is non-zero (so for planes not through the origin) the values for , and can be calculated as follows:
$a = \frac \begin 1 & y_1 & z_1 \\ 1 & y_2 & z_2 \\ 1 & y_3 & z_3 \end$
$b = \frac \begin x_1 & 1 & z_1 \\ x_2 & 1 & z_2 \\ x_3 & 1 & z_3 \end$
$c = \frac \begin x_1 & y_1 & 1 \\ x_2 & y_2 & 1 \\ x_3 & y_3 & 1 \end.$

These equations are parametric in ''d''. Setting ''d'' equal to any non-zero number and substituting it into these equations will yield one solution set.

Method 3

This plane can also be described by the prescription above. A suitable normal vector is given by the cross product
$\boldsymbol n = ( \boldsymbol p_2 - \boldsymbol p_1 ) \times ( \boldsymbol p_3 - \boldsymbol p_1 ),$
and the point can be taken to be any of the given points , or (or any other point in the plane).

Operations

Distance from a point to a plane

Line–plane intersection

Line of intersection between two planes

Sphere–plane intersection

Occurrence in nature

A plane serves as a mathematical model for many physical phenomena, such as specular reflection in a plane mirror or wavefronts in a traveling plane wave.
The free surface of undisturbed liquids tends to be nearly flat (see flatness).
The flattest surface ever manufactured is a quantum-stabilized atom mirror.
In astronomy, various ''reference planes'' are used to define positions in orbit.
''Anatomical planes'' may be lateral ("sagittal"), frontal ("coronal") or transversal.
In geology, ''beds'' (layers of sediments) often are planar.
Planes are involved in different forms of imaging, such as the ''focal plane'', ''picture plane'', and ''image plane''.

Miller indices

The attitude of a lattice plane is the orientation of the line normal to the plane,
and is described by the plane's Miller indices. In three-space a family of planes (a series of parallel planes) can be denoted by its Miller indices (''hkl''),

so the family of planes has an attitude common to all its constituent planes.

Strike and dip

Many features observed in geology are planes or lines, and their orientation is commonly referred to as their ''attitude''. These attitudes are specified with two angles.

For a line, these angles are called the ''trend'' and the ''plunge''. The trend is the compass direction of the line, and the plunge is the downward angle it makes with a horizontal plane.

For a plane, the two angles are called its ''strike (angle)'' and its ''dip (angle)''. A ''strike line'' is the intersection of a horizontal plane with the observed planar feature (and therefore a horizontal line), and the strike angle is the ''bearing'' of this line (that is, relative to geographic north or from magnetic north). The dip is the angle between a horizontal plane and the observed planar feature as observed in a third vertical plane perpendicular to the strike line.

See also

* Dihedral angle
* Flat (geometry)
* Half-plane
* Hyperplane
* Plane coordinates
* Plane of incidence
* Plane of rotation
* Plane orientation
* Polygon

Notes

Explanatory notes

Citations

References

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External links

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"Easing the Difficulty of Arithmetic and Planar Geometry"
is an Arabic manuscript, from the 15th century, that serves as a tutorial about plane geometry and arithmetic.

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