Polarization of light

As we've seen in the previous section an EM wave consists of oscillating electric and magnetic fields. In an EM wave these fields oscillate in a plane (plane of polarization) which is perpendicular to the plane in which the wave propagates (plane of propagation).

Electromagnetic wave propagating in space. The blue curve is meant to represent the oscillations of the Electric field while the yellow curve represents the Magnectic field. Polarization and propagation planes are indicated

An EM wave is said to be polarized if the oscillating Electric field "draws" a constant path on the plane of polarization while the wave is traveling.
The particular shape of this path identify the type polarization status. We can distinguish three major types

1) Elliptical polarization

2) Circular polarization

3) Linear Polarization

In the first case the figure that the Electric field "draws" in the plane of vibration is an ellipse. Circular polarization is just a particular case of elliptical polarization in which the major and the minor axes are equal. In the last case the electric field oscillates along a single line in the plane of polarization.

Linear and Circular polarization are represented in the above animated images. In the first case the electric field just propagates keeping a fixed direction and drawing a line on the Polarization plane. In the second case the wave also spins around itself drawing a circle on its polarization plane.

Natural light is unpolarized. Sources of EM waves like the sun or the common lamps give off light based on atomic excitation processes each one of them independent from the others. Because of this randomness, the resultant EM waves, which are going to be composed by the overlap of many of such elementary processes, will have a randomly oriented plane of vibration. Nevertheless it is always possible to get polarized light from unpolarized natural light.

Certain materials have the ability to filter out the unwanted oscillating directions of the electric field in an EM wave. Such a materials are called Polarizers. The effect of a polarizer is garantee from its particular microscopic structure.
Roughly speaking it is possible to imagine a polarizer as a material in which long chains of molecules have been aligned parallel to some particular direction using some manufacturing process. Such a material will absorb all the electric field components oscillating parallel to that direction and will transmit only the one which are perpendicular to it (axis of transmission of the polarizer).

Unpolarized light coming from the right hand side passes through a polarizer. Only the components of the field which oscillate along the polarizer transmission axis (the black line on the polarizer in the picture) will pass through it. Light coming out will be linearly polarized along that specific direction.

Let's suppose that we send a beam of unpolarized light through a pair of crossed polarizers (the transmission axis of the first polarizers is perpendicular to the transmission axis of the second). Light coming out from the first polarizer will be linearly polarized in a given direction. The beam then meets the second polarizer whose polarization axis is perpendicular to the one of the beam. The EM wave will be completely absorbed by the second polarizer and no light will make it through it. If the two polarizers were aligned in the same direction then all light coming out from the first polarizer would have passed the second polarizer untouched.

Unpolarized light coming from the right hand side gets stopped when passing through a pair of crossed polarizers.