Polarized light is light in which the electromagnetic wave vibrations are constrained to occur in a single defined plane perpendicular to the direction of propagation, rather than in the random mixture of all possible orientations perpendicular to the direction of travel that characterises ordinary unpolarized light. In unpolarized light, the electric field component of the electromagnetic wave vibrates in all directions at right angles to the direction of travel with equal probability and no preferred orientation, the random superposition of vibrations in every possible transverse direction producing the omnidirectional character of natural light from the sun, artificial lamps, and most other common light sources. In polarized light, this randomness is replaced by order - the electric field vibrations are restricted to a single plane, producing light with a specific and defined directional character that interacts differently with certain materials and optical elements than unpolarized light does.
Polarized light can be produced by several distinct physical mechanisms, each of which imposes order on the otherwise random orientations of light wave vibrations through a different physical process. Reflection from smooth non-metallic surfaces is one of the most commonly encountered sources of naturally polarized light in everyday photographic practice. When unpolarized light strikes a smooth dielectric surface - such as water, glass, polished wood, painted metal, or the waxy surface of foliage - the reflected light becomes partially or fully polarized parallel to the reflecting surface, with the degree of polarization reaching a maximum at the specific angle of incidence known as Brewster's angle for that material. This polarization of reflected light is the physical basis for the effectiveness of polarizing filters in controlling and eliminating unwanted specular reflections in photography.
Scattering of sunlight by molecules and particles in the Earth's atmosphere produces partially polarized light across areas of the sky, with the maximum degree of polarization occurring in regions of the sky approximately 90 degrees away from the direction of the sun. This atmospheric polarization is the reason why a polarizing filter darkens the blue sky most effectively when the camera is pointed at right angles to the sun, while having little or no darkening effect on sky areas directly towards or away from the sun where the scattered light is largely unpolarized.
Transmission through certain optically anisotropic materials - materials whose optical properties differ in different directions - can also produce polarized light through a process known as birefringence, in which the material splits an incident light beam into two components polarized at right angles to each other and transmits them at different speeds. This birefringent polarization is exploited in the construction of polarizing filters and in certain scientific instruments including polarimeters and polarising microscopes used in the analysis of crystalline materials and biological specimens.
In photographic applications, the interaction between polarized light and polarizing filters forms the basis of a range of important practical techniques. A polarizing filter placed over the camera lens and rotated to the appropriate angle selectively blocks the polarized component of reflected light while transmitting unpolarized light relatively freely, reducing or eliminating reflections from non-metallic surfaces, deepening the saturation of blue sky areas, and improving the overall colour saturation and contrast of outdoor images. In studio photography, crossed polarization techniques using polarized light sources and cross-polarized camera filters allow the complete elimination of all specular reflections from highly reflective subjects, revealing the true colour and surface detail that uncontrolled reflections would otherwise obscure.