Orthogonal Matrix

What is an Orthogonal Matrix?

An orthogonal matrix is a square matrix with real entries whose columns and rows are orthogonal unit vectors (orthonormal vectors). In other words, a matrix

Q is orthogonal if its transpose is equal to its inverse:

QT = Q-1

This property implies that multiplying an orthogonal matrix by its transpose results in the identity matrix:


where I is the identity matrix. The identity matrix is a special kind of diagonal matrix where all elements on the main diagonal are equal to 1, and all off-diagonal elements are 0.

Properties of Orthogonal Matrices

Orthogonal matrices have several important properties:

  • Preservation of the Dot Product: When an orthogonal matrix multiplies a vector, the dot product of the result with itself is the same as the dot product of the original vector with itself. This means that orthogonal transformations preserve lengths and angles, and hence are isometries of Euclidean space.
  • Preservation of Orthogonality: If two vectors are orthogonal, then after transformation by an orthogonal matrix, the resulting vectors will also be orthogonal.
  • Det(Q) = ±1: The determinant of an orthogonal matrix is always ±1. If the determinant is +1, the matrix is called a proper orthogonal matrix and represents a rotation; if it is -1, the matrix is called an improper orthogonal matrix and represents a reflection.
  • Invariance under Transposition: Since the transpose of an orthogonal matrix is also its inverse, the transpose of an orthogonal matrix is also orthogonal.
  • Stability under Multiplication: The product of two orthogonal matrices is also an orthogonal matrix.

Applications of Orthogonal Matrices

Orthogonal matrices are widely used in various fields of science and engineering. Some of the applications include:

  • Computer Graphics: In computer graphics, orthogonal matrices are used to represent rotations and reflections of objects in 3D space.
  • Signal Processing:

    In signal processing, orthogonal transformations such as the Fourier transform can be represented by orthogonal matrices, which are used for filtering and compressing signals.

  • Quantum Mechanics:

    In quantum mechanics, orthogonal matrices can represent the symmetries of a quantum system, and their eigenvalues are related to the conservation laws of the system.

  • Numerical Analysis:

    Orthogonal matrices play a crucial role in numerical linear algebra, particularly in algorithms for matrix factorization such as QR decomposition, which is used for solving linear systems and eigenvalue problems.

Constructing an Orthogonal Matrix

To construct an orthogonal matrix, one must ensure that the columns (and rows) of the matrix are orthonormal vectors. This can be achieved using the Gram-Schmidt process, which orthogonalizes a set of linearly independent vectors. The resulting vectors can then be normalized to have unit length, forming the columns of an orthogonal matrix.

Orthogonal Matrices in Linear Transformations

In the context of linear transformations, orthogonal matrices correspond to transformations that preserve the length of vectors, such as rotations and reflections. These transformations are particularly nice because they do not distort the shape of the objects being transformed.


Orthogonal matrices are a fundamental concept in linear algebra with wide-reaching implications in various scientific and engineering disciplines. Their properties, such as preserving the dot product and determinant values of ±1, make them invaluable tools for representing and performing isometric transformations, which include rotations and reflections that are ubiquitous in many applications.

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