Polarization of Waves

Polarization of waves

The directional dependence of certain wave phenomena, at right angles to the propagation direction of the wave. In particular, ordinary light may be regarded as composed of two such asymmetrical components, referred to as its two states of linear polarization.

These two components are refracted differently by doubly refracting crystals, such as calcite, or Iceland spar. Each state of linear polarization is refracted according to its own separate refractive index. On a subsequent refraction by the same crystal, but now rotated through an angle Θ about the direction of the beam, each component appears as a mixture of the original two polarization components, according to the proportions cos² Θ : sin² Θ. See Birefringence, Crystal optics, Refraction of waves

In the early nineteenth century, T. Young suggested that light polarization arises from transverse oscillations. In J. C. Maxwell's theory of light as electromagnetic waves, visible light---and also other types of electromagnetic radiation such as radio waves, microwaves, and x-rays (distinguished from visible light only by wavelength)---consists of electric and magnetic fields, each oscillating in directions perpendicular to the propagation direction, the electric and magnetic field vectors being perpendicular to each other. The plane of polarization of the wave contains the electric vector (or magnetic vector; there is no general agreement which) and the propagation direction. See Electromagnetic radiation, Light, Maxwell's equations

If the plane of polarization remains constant along the wave (as in the case of each light component in a doubly refracting medium), the wave has linear (or plane) polarization. However, the plane of polarization can also rotate. If the rotation rate is constant, the intensity of the wave being also constant, a circularly polarized wave results. These are of two types: right-handed and left-handed.

Any electromagnetic wave can be considered to be composed of monochromatic components, and each monochromatic component can be decomposed into a left-handed and a right-handed circularly polarized part. The states of linear polarization are each made up of equal magnitudes of the two circularly polarized parts, with differing phase relations to provide the different possible directions of plane polarization. Monochromatic waves composed of unequal magnitudes of the two circularly polarized parts are called elliptically polarized. This refers to the fact that the electric and magnetic vectors trace out ellipses in the plane perpendicular to the direction of motion.

Photons have quantum-mechanical spin, which refers to the angular momentum of the photon, necessarily about its direction of motion. A photon's spin has magnitude 1, in fundamental units. This spin can point along the direction of motion (positive helicity, right-handed spin) or opposite to it (negative helicity, left-handed spin), and this corresponds (depending on conventions used) to a classical electromagnetic wave of right- or left-handed circular polarization. See Helicity (quantum mechanics), Photon

Electromagnetic and gravitational waves both have the specific property that they are entirely transverse in character, which is a consequence of their speed of propagation being the absolute speed of relativity theory (the speed of light). This corresponds to the fact that their respective quanta, namely photons and gravitons, are massless particles. In the case of waves that travel at a smaller speed, as with fields whose quanta are massive rather than massless, there can be (unpolarized) longitudinal as well as transverse effects. Seismic waves traveling through the Earth's material, for example, can be transverse (polarized sideways oscillations) or longitudinal (unpolarized pressure waves). See Sound

In most situations encountered in practice, light (or gravitational waves) consists of an incoherent mixture of different polarization states, and is referred to as unpolarized. However, light reflected off a refracting surface (for example, glass or water) is polarized to some extent; that is, there is a certain preponderence of one state of linear polarization over the orthogonal possibility. Complete polarization occurs for a particular angle of incidence, known as the Brewster angle. See Polarized light, Reflection of electromagnetic radiation, Wave motion

Polarization of Waves