A polarizing filter, specifically the circular polarizer or CPL used on modern cameras, is an optical filter that selectively transmits light polarized in one orientation while blocking light polarized perpendicular to it. By rotating the filter ring relative to the lens, the photographer dials in the angle that maximizes or minimizes the effect. Properly aligned, a polarizer cuts reflections off non-metallic surfaces (water, glass, foliage, painted surfaces) and deepens blue skies in the part of the sky at right angles to the sun.
The physical basis is that light reflected off non-metallic surfaces becomes partially polarized in a horizontal plane. A filter aligned vertically blocks that reflected light while letting the rest of the scene through. Photographers point the front element at the surface they want to control (a pond, a car windshield, the wax sheen on a leaf), then rotate the filter while watching the viewfinder until the reflection disappears or reaches the desired strength. The effect is unique because no software can fully replicate it: once polarized highlights are recorded into the file, they cannot be cleanly separated from the underlying subject. You either capture the polarization in-camera or you do not.
The blue-sky effect comes from the same physics. Skylight is polarized most strongly 90 degrees from the sun. A polarizer rotated to block that polarized blue light darkens the sky relative to clouds and ground, producing the dramatic deep blue with bright white cumulus that defines classic landscape photography. The effect is strongest with a 90-degree sun angle and falls off as the sun moves directly overhead or in line with the lens. With wide-angle lenses (wider than about 24 mm on full frame), the polarized band crosses only part of the frame, producing a visibly uneven sky that often looks worse than no polarization.
Polarizers cost light, typically 1.3 to 2 stops, so they double as mild ND filters in bright conditions. They also saturate foliage by removing the surface reflection from leaves and revealing the underlying chlorophyll color. For waterfall and stream photography, polarizers reveal river-bottom detail otherwise hidden by glare. For shooting through windows (storefronts, car interiors, museum cases), a polarizer can almost eliminate reflections at the right angle.
The distinction between circular and linear polarizers matters for autofocus and metering. Linear polarizers interact with the phase-detect AF beam splitter in DSLRs and can confuse metering sensors. Circular polarizers add a quarter-wave plate behind the polarizing element that scrambles the polarization before it reaches the AF and metering modules, restoring normal camera operation. All modern photo polarizers are circular for this reason; linear polarizers persist only for specialized scientific and cinematography uses.
Common pitfalls include over-polarizing skies into unnatural near-black, polarizing the rainbow out of waterfalls and oil slicks (the rainbow comes from polarized reflections), and stacking the CPL with other filters at wide angles where vignetting becomes severe. Quality glass matters: cheap polarizers introduce color casts and reduce lens sharpness. B+W, Hoya HD, Nisi, Breakthrough, and Lee are dependable brands across price ranges.