Polaroid sheets work because stretched plastic lines up iodine electrons
The polarising film in sunglasses and phone screens is a sheet of polyvinyl alcohol plastic laced with iodine. Stretching it during manufacture lines its long molecules up in one direction, so electrons can slide along them but not across. Light vibrating along the chains gets absorbed; light vibrating crosswise passes through.
A polariser lets through light waves vibrating in one particular orientation and blocks the rest, turning a jumble of mixed light into a well-ordered beam. Photographers use polarising filters to suppress reflections, and every liquid-crystal display depends on them. The same principle can be applied to radio waves, microwaves and even X-rays.
Some crystals polarise naturally by absorbing light that vibrates in certain directions, an effect called dichroism. Tourmaline is the best known, but it looks coloured and its behaviour varies strongly with wavelength, so it is rarely used. Herapathite is nearly colourless but hard to grow into large crystals. The original Polaroid film was made from microscopic herapathite crystals before switching to the iodine-doped plastic now standard. Being cheap and tough, it is by far the most common polariser. A newer rival embeds elongated silver nanoparticles in thin glass, blocking the unwanted orientation up to 100,000 times better than it passes the wanted one; these excel in infrared and are widely used in fibre-optic networks.
Other polarisers split light rather than absorbing it, sending the two orientations off as separate beams. Because they do not have to soak up and dissipate the rejected energy, they suit intense sources such as lasers, and they are useful when both halves of the light need analysing.
Plain glass can polarise too. At a specific tilt called Brewster's angle, about 57 degrees for ordinary glass in air, reflected light contains only one orientation. A stack of tilted glass plates therefore strips away that orientation little by little, each surface reflecting roughly 16 percent of it. The method is inefficient: even ten plates, with twenty reflecting surfaces, still let about 3 percent through.
Source: Polarizer
