polarization

Light is a transverse wave, its electric field wiggles crosswise to the direction it travels, but crosswise which way, up-down, left-right, or something in between? Polarization is the answer, it describes the direction along which a light wave's field oscillates. Ordinary sunlight or lamplight is unpolarized, a jumble of all these directions mixed together, while a laser or light through a polaroid filter can be lined up in a single direction. It answers the question: transverse to travel is a whole plane of possibilities, so which way, exactly, is the wave shaking?

Because light is a transverse electromagnetic wave, its electric field points at right angles to the direction of travel, but that still leaves a full circle of possible orientations. Linearly polarized light has its field oscillating along one fixed line. A polarizing filter (a polaroid) acts like a picket fence for waves, it transmits only the field component along its transmission axis and absorbs the rest. When polarized light of intensity I_0 meets a filter whose axis is at angle theta to the light's polarization, the transmitted intensity follows Malus's law, I = I_0 cos^2(theta): aligned filters pass everything, crossed filters (theta = 90 degrees) pass nothing and go dark. Light also becomes partly polarized when it reflects off a surface like water or glass at a glancing angle.

Polarization is put to everyday use: polarizing sunglasses cut the horizontally polarized glare bouncing off roads and water, and LCD screens work by rotating polarized light between crossed filters to make pixels bright or dark, which is why looking at a phone through polarized sunglasses can turn the screen black at certain angles. Crucially, only transverse waves can be polarized, so the fact that light can be polarized is direct evidence that it is transverse. An honest contrast: sound is a longitudinal wave, its oscillation is along the direction of travel with no crosswise choice to make, so sound cannot be polarized at all.