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Light is a transverse electromagnetic wave in which the electric field oscillates perpendicular to propagation. Unpolarized light has electric field vectors in all transverse directions equally. A polarizer transmits only the component of E along its transmission axis, producing linearly polarized light. Polarization is exclusive to transverse waves — longitudinal waves like sound cannot be polarized. Methods of polarizing light include selective absorption (Polaroid filters), reflection (at Brewster's angle), and scattering.
Cross two polarizing filters completely to block all light, then insert a third at 45°. The surprising reappearance of light demonstrates that polarization states add vectorially, not as simple on/off filters.
You already know that light is a transverse electromagnetic wave — the electric and magnetic fields oscillate perpendicular to the direction the wave travels. "Perpendicular to the direction of travel" describes a whole plane, and in unpolarized light, the electric field oscillates in every direction within that plane simultaneously and randomly. Think of it as a bundle of arrows all pointing outward from the wave's travel axis, randomly changing orientation many billions of times per second. Polarization means restricting those oscillations to a single direction, or a predictable pattern of directions.
The simplest way to polarize light is a polarizer — a material (like Polaroid film) that has an aligned molecular structure that absorbs electric field components perpendicular to its transmission axis while letting the parallel component pass. When unpolarized light hits a polarizer, only the component of each randomly oriented electric field vector that lies along the transmission axis survives. The output is light with the electric field oscillating exclusively along one direction: linearly polarized light. Because the random incoming vectors project onto the transmission axis, on average half the intensity passes through — this is why sunglasses dim the world while eliminating glare.
Polarization by reflection works differently and connects to Snell's law and electromagnetic boundary conditions. When light strikes a surface at a specific angle called Brewster's angle, the component of the electric field oscillating in the plane of incidence (the "p-polarization") is not reflected at all — it is entirely transmitted. Only the s-polarization (electric field perpendicular to the plane of incidence) reflects. The reflected glare from water, roads, and windows is therefore partially or fully s-polarized, which is why polarizing sunglasses — with their transmission axis vertical — cut glare selectively: they absorb the horizontally oscillating, reflected s-polarization.
The most counterintuitive result in introductory polarization is the three-polarizer experiment. Two crossed polarizers (transmission axes at 90°) block all light: the second polarizer's axis is perpendicular to the first, so zero component of the polarized light from the first survives. But insert a third polarizer *between* them at 45°, and light gets through. The first polarizer produces vertically polarized light. The middle polarizer at 45° passes the component of that vertical field along its 45° axis — reducing intensity by cos²(45°) = 50%, but now the light leaving the middle polarizer is polarized at 45°. The final polarizer, originally perpendicular to the first polarizer but at 45° to the middle one, passes the cos²(45°) = 50% component of the 45°-polarized light. The three-polarizer result isn't magic; it's a reminder that polarizers don't just block light — they *reorient* the polarization state, and it is the new state that encounters the next polarizer.