Polarization is a fundamental property of light that describes the orientation of the transverse wave vibrations that constitute a light beam as it travels through space. In ordinary unpolarized light - such as that emitted by the sun, artificial lamps, or reflected from most surfaces - the electromagnetic vibrations that make up the light wave occur in all possible orientations perpendicular to the direction of travel simultaneously, with no preferred direction of vibration. Polarized light, by contrast, is light in which the electromagnetic vibrations are constrained to occur in a single plane or in a defined rotational pattern, rather than in the random mixture of all orientations that characterises natural unpolarized light.
Polarization of light can occur naturally through several physical mechanisms. Reflection from smooth, flat non-metallic surfaces - such as water, glass, polished wood, painted surfaces, and foliage - partially or completely polarizes the reflected light, with the degree of polarization depending on the angle of incidence and the refractive index of the reflecting material. At a specific angle of incidence known as Brewster's angle - which varies with the refractive index of the material but is typically around 53 to 57 degrees for common materials such as water and glass - the reflected light is completely polarized in the plane parallel to the reflecting surface. Scattering of sunlight by atmospheric particles also produces partially polarized light, with the maximum polarization occurring in portions of the sky at 90 degrees to the direction of the sun.
Polarization in photography is most commonly introduced and controlled using a polarizing filter - a sheet of optically clear material containing a layer of aligned molecular or crystalline structure that selectively absorbs light vibrating in one orientation while transmitting light vibrating in the perpendicular orientation, effectively passing only the component of any incident light that vibrates in the transmission direction of the filter. When a polarizing filter is placed over a camera lens and rotated to the appropriate angle, it selectively blocks the polarized component of light reflected from non-metallic surfaces, reducing or completely eliminating the specular reflections that would otherwise obscure the colour and detail of the surface beneath. The same filter simultaneously darkens blue sky areas of the image by blocking the partially polarized scattered skylight, increasing the contrast between the blue sky and white clouds and producing the deep, saturated blue sky rendition that is one of the most familiar and popular applications of polarizing filters in outdoor photography.
Polarizing filters are also used over artificial light sources in studio photography - positioned over the light source rather than the lens - in combination with a polarizing filter over the camera lens. When the two filters are oriented with their transmission axes at 90 degrees to each other in a crossed polarization arrangement, virtually all specular reflections are eliminated from the subject, as the polarized light from the source that would produce specular reflections cannot pass through the cross-polarized camera filter. This technique is particularly valuable in product and scientific photography where the complete elimination of all surface reflections is required to reveal the true colour, texture, and detail of highly reflective subjects.
Two types of polarizing filter are available for photographic use, differing in the optical arrangement of their polarizing layers. Linear polarizing filters produce linearly polarized light in which all vibrations occur in a single plane, and were the original and simplest type of polarizing filter used in photography. However, linear polarizers can interfere with the beam splitting elements used in the through the lens metering and autofocus systems of modern SLR and mirrorless cameras, causing inaccurate meter readings and unreliable autofocus performance. Circular polarizing filters address this problem by adding a quarter wave retarder element after the linear polarizing layer, which converts the linearly polarized light to circularly polarized light before it enters the camera, preserving the full effectiveness of the polarizing effect on reflections while eliminating the interference with the camera's internal optical systems.