Common sources of light, such as the Sun and the electric light bulb emit what is known as unpolarised light. More specialised sources, such as certain kinds of discharge tubes and lasers, produce polarised light. The difference between these two types of light is caused by the behaviour of the electromagnetic fields that make up the light.
As decribed by Maxwell's equations, light is a transverse wave made up of an interacting electric field E and a magnetic field B. The oscillations of these two interacting fields cause the fields to self-propagate in a certain direction, at the speed of light. In most cases, the directions of the electric field, the magnetic field, and the direction of propagation of the light are all mutually perpendicular. That is, Both the E and B fields oscillate in a direction at right angles to the direction that the light is moving, and also at right angles to each other.
(In optics, it is usual to define the polarisation in terms of the direction of the electric field, and disregard the magnetic field since it is almost always perpendicular to the electric field.)
If the direction of oscillation of the electric field E is fixed, the light wave is said to be linearly polarised. There are two possible linear polarisation states, with their E fields orthogonal to one another. Any other angle of linear polarisation can be constructed as a superposition of these two states.
The direction of polarisation is arbitrary with respect to the light itself. It is usual to label the two linear polarisation states in accordance with some other external reference. For example, the terms horizontally and vertically polarised are generally used when light is propagating in free space. If the light is interacting with a surface, such as a mirror, lens or some other interface between two media, the terms s- and p-polarised are used. For example, consider the following:
\ ' / \ ' / \ ' / \ ' / \ ' / \ ' / \ ' / \'/ ======================
In the above diagram, a light ray is reflecting off a mirror at some angle. If the electric field of the light is oscillating perpendicular to the plane of the diagram, the light is termed s-polarised. If it is oscillating in the plane of the diagram, it is termed p-polarised. Other terms used for s-polarisation are sigma-polarised and sagittal plane polarised. Similarly, p-polarised light is also refered to as pi-polarised and tangential plane polarised.
If the direction of the electric field E is not fixed, but rotates as the light propagates, the light is said to be circularly polarised. Two possible independent circular polarisation states exist, termed left-hand or right-hand circularly polarised depending on whether the electric field is rotating in a counter-clockwise or clockwise sense, respectively, when looking in the direction of the light propogation.
If the light consists of many incoherent waves with randomly varying polarisation, the light is said to be unpolarised. It is possible to convert unpolarised light to polarised light by using a polariser. One such device is Polaroid® sheet. This is a sheet of plastic with molecules that are arranged such that they absorb any light passing through in which has an electric field oscillating in a given direction; This has the effect of linearly polarising the light. Other devices can split an unpolarised beam into two beams of orthogonal linear polarisation; They are generally constructed from certain arrangements of prisms and optical coatings.
The angle of polarisation of linearly polarised light can be rotated using a device known as a half-wave plate. Similarly, linear polarisation can be converted to circular polarisation and vice versa with the use of a quarter-wave plate.
In electromagnetism, the polarisation is also the name given to the vector field that results from permanent or induced dipole moments in a dielectric material. The polarisation vector P is defined as the dipole moment per unit volume.
- P = ε0χE ,
where ε0 is the permittivity of free space, and χ is the electric susceptibility of the medium.
If the polarisation P is not proportional to the electric field E, the medium is termed nonlinear and is described by the field of nonlinear optics. If the direction of P is not aligned with E, as in many crystals, the medium is anisotropic and is described by crystal optics.