Polarisation is a property of electromagnetic radiation that describes the orientation and behaviour of the electric field associated with a propagating electromagnetic wave. For a simple plane electromagnetic wave travelling in a particular direction, the electric field oscillates in a direction perpendicular to the direction of propagation. Polarisation specifies how that electric field vector changes with time at a fixed location. If the electric field remains aligned along one fixed transverse direction, the wave is linearly polarised.
If the electric field rotates while maintaining constant magnitude, the wave can be circularly polarised. More generally, the electric field can trace an ellipse, producing elliptical polarisation. Polarisation is therefore not simply a statement that light travels in one direction. It concerns the orientation and temporal evolution of the transverse electromagnetic field. Because ordinary visible light contains electromagnetic waves, polarisation is an intrinsic property of light. The same principle applies to other electromagnetic radiation, including radio waves, microwaves, infrared radiation, ultraviolet radiation, X rays and gamma rays, although the methods used to generate, measure and manipulate polarisation vary considerably with frequency.
Linear polarisation occurs when the electric field oscillates along a single fixed axis perpendicular to propagation. A polarising filter can preferentially transmit one component of an incident electromagnetic field while attenuating the orthogonal component. When linearly polarised light passes through an ideal linear polariser, the transmitted intensity depends on the angle between the incident polarisation direction and the transmission axis.
For ideal conditions, Malus law states that transmitted intensity is proportional to the square of the cosine of the angle between these axes. This provides a practical method for measuring polarisation and illustrates why two polarisers with perpendicular transmission axes can strongly suppress transmitted light. A polariser does not necessarily create all the polarisation properties of the incident field from nothing. Instead, it selectively transmits or absorbs field components. Real polarisers have finite extinction ratios, meaning unwanted polarisation components are reduced but not necessarily eliminated. Polarising materials, birefringent crystals, wire grid structures and specialised optical coatings can all perform polarisation selection at appropriate wavelengths.
Circular and elliptical polarisation arise when two perpendicular electric field components have suitable amplitudes and a relative phase difference. Circular polarisation occurs when the two orthogonal components have equal amplitudes and a phase difference of ninety degrees, with the field vector rotating at a constant magnitude. The direction of rotation determines whether the polarisation is described as right handed or left handed, although exact handedness conventions depend on the viewing and propagation convention being used.
Elliptical polarisation is the general case in which the field traces an ellipse. Linear polarisation can be regarded as a special case of elliptical polarisation in which the ellipse collapses into a line, while circular polarisation is the special case in which the ellipse becomes a circle. Optical components called waveplates exploit controlled phase delays between orthogonal components to transform one polarisation state into another. A quarter wave plate can convert suitable linear polarisation into circular polarisation and can also convert circular polarisation into linear polarisation, while a half wave plate can rotate linear polarisation.
Polarisation is closely related to the electromagnetic nature of light. Maxwell's equations predict transverse electromagnetic waves in which electric and magnetic fields are mutually perpendicular and both are perpendicular to the direction of propagation for a simple plane wave in free space. The electric and magnetic fields are linked, so specifying the electric field polarisation also determines the corresponding orientation of the magnetic field.
In isotropic materials, different polarisation components may propagate similarly, but in anisotropic materials their propagation can differ. Birefringent materials have refractive indices that depend on the orientation of the electromagnetic field relative to the material's internal structure. This can cause different polarisation components to accumulate different phases as they travel. Polarisation effects are consequently important in mineral identification, microscopy, spectroscopy, telecommunications, astronomy and precision optical measurement. The mathematical description of polarisation commonly uses Jones vectors and matrices for fully coherent polarised light, while Stokes parameters and Mueller matrices are useful for more general partially polarised light and for describing optical systems that may depolarise radiation.
Natural light sources often produce light that is unpolarised or only partially polarised because many independent emitting atoms or molecules radiate with different orientations and phases. Sunlight can become partially polarised when scattered by the atmosphere, and reflection from surfaces can preferentially reflect one polarisation component. This effect is described by Fresnel equations and is especially pronounced near Brewster's angle, where reflected light from a dielectric interface can become strongly linearly polarised.
Polarisation by reflection explains why polarising sunglasses can reduce certain forms of glare. The polarising material is oriented to attenuate a selected component of the reflected light. Polarisation can also arise from scattering, birefringence, selective absorption and emission mechanisms. Astronomers use polarisation measurements to investigate magnetic fields, dust grains, scattering environments and other physical processes that may not be obvious from intensity measurements alone. Thus, polarisation carries information about how radiation interacted with matter on its journey from source to detector.
In optical communications, polarisation can be both useful and problematic. Light travelling through optical fibre can have different polarisation states, and environmental effects can cause the polarisation state to change with time. Fibre birefringence can result from manufacturing imperfections, mechanical stress, bending and temperature changes. In high capacity coherent communication systems, polarisation can be deliberately treated as an additional degree of freedom, allowing two independent data streams to be transmitted using two orthogonal polarisation states.
Receivers can then estimate and compensate for changes introduced by the fibre. Polarisation mode dispersion can occur when different polarisation modes propagate at slightly different group velocities, potentially limiting system performance at very high data rates. Optical engineers therefore characterise polarisation properties carefully when designing lasers, fibres, modulators, detectors and network components. The ability to control polarisation is a major part of modern photonics and is essential in many precision optical systems.
Polarisation also has important applications outside conventional imaging and communication. In liquid crystal displays, electrically controlled molecular orientation changes the optical properties of the material and controls how polarised light is transmitted through the display. In microscopy, crossed polarisers and birefringent specimens can reveal structures that are difficult to observe using ordinary illumination. In remote sensing, polarisation measurements can provide information about aerosols, clouds, surfaces and biological materials.
In radar and radio communication, polarisation is used to distinguish signals, manage propagation effects and optimise antenna performance. Circularly polarised antennas can be useful in satellite communication because the relative orientation between transmitter and receiver may vary. Polarisation can also be used in three dimensional imaging systems, interferometers, optical isolators and quantum information experiments. In quantum physics, polarisation of a photon provides a two state or higher dimensional degree of freedom that can be used to encode quantum information, although classical polarisation theory remains sufficient for many conventional optical applications.
A key distinction is that polarisation is not the same thing as colour, intensity or direction of propagation. Colour is primarily associated with wavelength or frequency, intensity describes optical power or energy flux, and propagation direction describes where the wave travels. A single wavelength can possess many polarisation states, and the same polarisation state can occur across a range of wavelengths. Polarisation is also distinct from coherence, although coherent sources make polarisation easier to describe using field amplitudes and phases.
A beam can be partially polarised, meaning that its radiation contains both an organised polarised component and an unpolarised component. Real optical systems can alter polarisation through reflection, transmission, absorption, scattering, birefringence and differential phase delay. Understanding these distinctions prevents common misconceptions such as treating polarisation as an intrinsic colour property or assuming that all light is either completely polarised or completely unpolarised. Polarisation is instead a precise electromagnetic property describing the transverse electric field structure of radiation and providing a powerful way to generate, manipulate and extract information from electromagnetic waves.
