Polarized Light

Polarized light is light whose electric field oscillates in a preferred orientation rather than evenly across every angle perpendicular to its travel. Thermal sources emit unpolarized light; reflection from a non-metallic surface, scattering in the atmosphere, and a polarizing filter each impose an orientation. Polarization belongs to any electromagnetic wave, radio included, which is why antenna orientation matters in Specific Absorption Rate testing.

A linear polarizer passes only the field component parallel to its transmission axis. Malus's law gives the result: transmitted intensity equals incident intensity times cos²θ, where θ is the angle between the incoming polarization and the axis. Ideal polarizers crossed at 90° pass nothing; at 45° they pass half. Glare off water and roads is strongly horizontally polarized, so sunglasses with a vertical axis remove it while barely dimming the rest of the scene.

An LCD sits between two linear polarizers: the crystal layer rotates polarization by an amount the front polarizer converts into brightness. Its output is linearly polarized by construction, so a filter at the right angle extinguishes it. An OLED emits its own light through a circular polarizer — a linear layer plus a quarter-wave retarder — bonded on to cancel ambient reflection off the metal cathode, and that outer axis is commonly set at 45° to the screen edges, so neither portrait nor landscape blacks it out. Behaviour varies by panel: construction, not category, is the Ground Truth.

Crossed polarizers also reveal stress birefringence: strained plastics and toughened glass split light into two components of differing speed, producing coloured fringes that map the stress field — coloured because the retardation depends on wavelength, a form of Dispersion.