Plane (geometry)


In mathematics, a plane is a flat, two-dimensional surface that extends infinitely far. A plane is the two-dimensional analogue of a point, a line and three-dimensional space. Planes can arise as subspaces of some higher-dimensional space, as with a room's walls extended infinitely far, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry.
When working exclusively in two-dimensional Euclidean space, the definite article is used, so the plane refers to the whole space. Many fundamental tasks in mathematics, geometry, trigonometry, graph theory, and graphing are performed in a two-dimensional space, or, in other words, in the plane.

Euclidean geometry

set forth the first great landmark of mathematical thought, an axiomatic treatment of geometry. He selected a small core of undefined terms and postulates 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. In this way the Euclidean plane is not quite the same as the Cartesian plane.
A plane is a ruled surface.

Representation

This section is solely concerned with planes embedded in three dimensions: specifically, in R3.

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:
The following statements hold in three-dimensional Euclidean space but not in higher dimensions, though they have higher-dimensional analogues:
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 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
Expanded this becomes
which is the point-normal form of the equation of a plane. This is just a linear equation
where
Conversely, it is easily shown that if and are constants and, and are not all zero, then the graph of the equation
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.
Thus for example a regression equation of the form 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
where s and t range over all real numbers, ' and are given linearly independent vectors defining the plane, and is the vector representing the position of an arbitrary 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 that satisfy the following determinant equations:

Method 2

To describe the plane by an equation of the form, solve the following system of equations:
This system can be solved using Cramer's rule and basic matrix manipulations. Let
If D is non-zero the values for a, b and c can be calculated as follows:
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 "point and a normal vector" prescription above. A suitable normal vector is given by the cross product
and the point can be taken to be any of the given points, or .

Operations

Distance from a point to a plane

For a plane and a point not necessarily lying on the plane, the shortest distance from to the plane is
It follows that lies in the plane if and only if D=0.
If meaning that a, b, and c are normalized then the equation becomes
Another vector form for the equation of a plane, known as the Hesse normal form relies on the parameter D. This form is:
where is a unit normal vector to the plane, a position vector of a point of the plane and D0 the distance of the plane from the origin.
The general formula for higher dimensions can be quickly arrived at using vector notation. Let the hyperplane have equation, where the is a normal vector and is a position vector to a point in the hyperplane. We desire the perpendicular distance to the point. The hyperplane may also be represented by the scalar equation, for constants. Likewise, a corresponding may be represented as. We desire the scalar projection of the vector in the direction of. Noting that we have

Line–plane intersection

In analytic geometry, the intersection of a line and a plane in three-dimensional space can be the empty set, a point, or a line.

Line of intersection between two planes

The line of intersection between two planes and where are normalized is given by
where
This is found by noticing that the line must be perpendicular to both plane normals, and so parallel to their cross product .
The remainder of the expression is arrived at by finding an arbitrary point on the line. To do so, consider that any point in space may be written as, since is a basis. We wish to find a point which is on both planes, so insert this equation into each of the equations of the planes to get two simultaneous equations which can be solved for and.
If we further assume that and are orthonormal then the closest point on the line of intersection to the origin is. If that is not the case, then a more complex procedure must be used.

Dihedral angle

Given two intersecting planes described by and, the dihedral angle between them is defined to be the angle between their normal directions:

Planes in various areas of mathematics

In addition to its familiar geometric structure, with isomorphisms that are isometries with respect to the usual inner product, the plane may be viewed at various other levels of abstraction. Each level of abstraction corresponds to a specific category.
At one extreme, all geometrical and metric concepts may be dropped to leave the topological plane, which may be thought of as an idealized homotopically trivial infinite rubber sheet, which retains a notion of proximity, but has no distances. The topological plane has a concept of a linear path, but no concept of a straight line. The topological plane, or its equivalent the open disc, is the basic topological neighborhood used to construct surfaces classified in low-dimensional topology. Isomorphisms of the topological plane are all continuous bijections. The topological plane is the natural context for the branch of graph theory that deals with planar graphs, and results such as the four color theorem.
The plane may also be viewed as an affine space, whose isomorphisms are combinations of translations and non-singular linear maps. From this viewpoint there are no distances, but collinearity and ratios of distances on any line are preserved.
Differential geometry views a plane as a 2-dimensional real manifold, a topological plane which is provided with a differential structure. Again in this case, there is no notion of distance, but there is now a concept of smoothness of maps, for example a differentiable or smooth path. The isomorphisms in this case are bijections with the chosen degree of differentiability.
In the opposite direction of abstraction, we may apply a compatible field structure to the geometric plane, giving rise to the complex plane and the major area of complex analysis. The complex field has only two isomorphisms that leave the real line fixed, the identity and conjugation.
In the same way as in the real case, the plane may also be viewed as the simplest, one-dimensional complex manifold, sometimes called the complex line. However, this viewpoint contrasts sharply with the case of the plane as a 2-dimensional real manifold. The isomorphisms are all conformal bijections of the complex plane, but the only possibilities are maps that correspond to the composition of a multiplication by a complex number and a translation.
In addition, the Euclidean geometry is not the only geometry that the plane may have. The plane may be given a spherical geometry by using the stereographic projection. This can be thought of as placing a sphere on the plane. This is one of the projections that may be used in making a flat map of part of the Earth's surface. The resulting geometry has constant positive curvature.
Alternatively, the plane can also be given a metric which gives it constant negative curvature giving the hyperbolic plane. The latter possibility finds an application in the theory of special relativity in the simplified case where there are two spatial dimensions and one time dimension.

Topological and differential geometric notions

The one-point compactification of the plane is homeomorphic to a sphere ; the open disk is homeomorphic to a sphere with the "north pole" missing; adding that point completes the sphere. The result of this compactification is a manifold referred to as the Riemann sphere or the complex projective line. The projection from the Euclidean plane to a sphere without a point is a diffeomorphism and even a conformal map.
The plane itself is homeomorphic to an open disk. For the hyperbolic plane such diffeomorphism is conformal, but for the Euclidean plane it is not.