Polarization

Polarization (also polarisation) is a property of some types of waves. Some waves have motion in another direction as well as the direction of the wave. In water, for instance, the wave will move across the water, but something on the surface of the water will move up and down. This is not a perfect example because the water itself isn't moving forward, and polarization is for particles that do move forward. A better example is a spot on a car tire - as it moves forward, it also moves up and down.

Sounds waves do not have polarization, since they only move in one direction.

Light

Light has polarization, and it is always perpendicular to the forward direction, this is called a transverse wave (like the spot on a car tire).

Polarization Filter

Polarization of light is useful, after it has been filtered. A filter will separate light with one type of polarization from other types. Most daylight, or light from a light bulb, has a mix of polarizations (lasers are an exception). The filter works much like trying to put a playing card through a comb - only if the card is turned the right direction will it fit. Light that is turned a different way will be blocked by the filter. Liquid crystal displays (LCD's) use this to block light to make letters or numbers on a display. Glasses that have different polarization filters for each eye can separate light meant for the left eye and for the right eye. This is a common way to make 3D movies and 3D television.

In nature sometimes light that bounces off of a surface has the same polarization - this is called 'glare' from a window or water. A polarization filter on a camera will remove this glare to help see through the window or water (or can increase the glare, depending on how it is turned).

Electromagnetic

Electromagnetic waves also have polarization, but it can happen in a couple different ways.

General polarization: Some waves can be described by having the electric field perpendicular to the direction of the wave, and these are called TE (transverse electric) waves. Others have the magnetic field perpendicular to the direction of the wave, and these are called TM (transverse magnetic) waves. These are the most general types of wave polarization. They can also be called vertical or horizontal polarized waves. If both the electric field and magnetic field are perpendicular to the direction of the wave, the wave is called TEM (transverse electromagnetic). Linear, circular, and elliptical polarizations are specific cases of TEM polarization.

Linear, circular, and elliptical polarizations are three specific types of TEM polarization. They cannot be measured close to an antenna. Far away from an antenna the fields are TEM, so these can be used. They are easy to imagine if you look from at the wave straight on.

Linear polarization

Looking along the direction of the wave, the electric field is a single straight line. If the direction of the electric field remains constant, even if the size or magnitude is allowed to change, the state of polarization is referred to as linear. This is because the tip of the electric field vector traces out a straight line in the plane perpendicular to the forward direction of the wave. (Similar to shaking a jump rope up and down, looking at the rope from one end. The moving rope will make the shape of a line).

Circular polarization

Looking along the direction of the wave, the electric field traces out the shape of a circle. If the magnitude of the electric field remains constant, and the direction is allowed to vary, the state of polarization is referred to as circular, because the tip of the electric field traces out a circle in the plane perpendicular to the forward direction of the wave. In particular, the circle may be traced out in a clockwise or counterclockwise direction (as the wave is traveling away). (This is similar to swinging a jump rope around for jumping, it makes a circle shape).

Elliptical polarization

This is like the circular polarization, but the magnitude changes as it rotates, forming an ellipse.