Polarization

The polarization of light plays a decisive role in many applications. In this article you will learn among other things what polarization is and how it can be generated.

You are more the audiovisual type of learner? For polarization we have an animated Video , by explaining the most important things from this post in a short time.

• Polarization simply explained
• Polarization of light
• Generation of polarization states
• Polarization Examples

Polarization explained simply

Our initial situation is that in which we consider light as a wave. More precisely we see light as a electromagnetic wave at. Similar to water waves, light is a Transverse wave, where in an electromagnetic wave not particles oscillate up and down but electric and magnetic fields. For this reason, polarization can be defined for light.

The Polarization of light describes the direction of oscillation of the electric field vector.

To give you a more descriptive example, you can imagine rope waves. You can swing the rope up and down, from left to right, or in any other oblique direction. The direction in which you swing the rope determines how the particles of the rope will swing. If you now look at the rope shaft from the front, you will recognize a straight line depending on the direction of oscillation.

For example, the horizontal direction of oscillation is the one you get by swinging the rope from left to right, while an oblique one is the one you get by swinging the rope at an angle. And exactly this direction of oscillation is called polarization.

polarization of light

For the definition of polarization must a transversal wave is present. In this section, we explain what a transverse wave is, how it differs from a longitudinal wave, and what types of polarization exist.

Transverse vs. Longitudinal waves

The distinction between Transverse waves and longitudinal waves is based solely on the position of the propagation direction of the wave and the deflection of the particles (in the case of light the oscillation direction of the electric field).

In transverse waves, the deflection of the particles (resp. the direction of oscillation of the electric field) perpendicular for the propagation direction of the wave. So when the wave moves from left to right, the particles oscillate up and down. Examples for transversal waves are electromagnetic waves and water waves.

In contrast to this, the deflection of a longitudinal wave is parallel On the direction of propagation of the wave. Examples are sound waves and earthquake waves (also called seismic waves).

The difference between these two types of waves should now show you better why polarization can only be defined for transverse waves. While there are many ways for the deflection to be perpendicular to the direction of propagation, there is only one way for the deflection to be parallel to the direction of propagation. Therefore it would make no sense at all to talk about polarization in case of longitudinal waves.

Polarization types

The different types of polarization, depending on the direction of oscillation of the electric field and its magnitude, are divided into

• linear polarizationThe direction of oscillation of the electric field is constant, but its magnitude changes periodically.
• circular polarizationHere the magnitude of the electric field is constant, but its direction of oscillation changes with a fixed angular velocity .
• elliptical polarizationIn this type of polarization, both the magnitude of the electric field and its direction of oscillation change.

The naming for the polarization types comes from the fact that when viewed from the front, the electric field vector travels along the following geometric shapes

In linear polarization, for example, the electric field vector moves along a line, while in circular polarization it moves along a circle.

Generation of polarization states

The light of the sun is unpolarized. This means that the electric field vector changes arbitrarily in its direction of oscillation and its magnitude. There are several ways to get polarized light from unpolarized light. These methods are presented in this section.

polarization by absorption

One method to obtain linearly polarized light is by using a Polarization filter. A polarizing filter consists of a large number of molecules arranged parallel to each other. If an electromagnetic wave hits this "molecular wall", the part of the electric field which is parallel to the molecules can set the electrons of the molecules in motion. This causes the field to do work on the molecules, allowing them to absorb its energy. The component perpendicular to the molecules, on the other hand, does not have the chance to do any work on the molecules. As a result, this part cannot transfer its energy and passes through the molecular wall with almost no losses.

The direction of oscillation that can pass through the polarization filter without absorption defines the Axis of the polarization filter. The axis of a polarizing filter is therefore perpendicular to the orientation of the molecules.

polarization by reflection

Another way to obtain polarized light is the polarization by reflection. In this case the electric field of unpolarized light meets an interface between two meids with different refractive indexes . This electric field can be divided into a component parallel to the plane of incidence and one perpendicular to it. The component parallel to the plane of incidence accelerates the electrons at the interface also parallel to it. Now it is a result from electrodynamics that accelerated electrons in turn emit radiation in such a way that no radiation is emitted along their acceleration direction. The parallel component of the electric field can therefore not be in the reflected part of the wave. The reflected light is thus partially polarized.

At a very specific angle, the so-called Polarization angle (or Brewster angle) the reflected light is even completely polarized. Experimentally, it was found that the reflected beam is then perpendicular to the refracted beam. You can calculate this polarization angle for given refractive indices and with the help of the Brewster’s law as follows

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polarization by scattering

The last possibility we want to discuss is the Polarization through dispersion. Here, the electric field of unpolarized light strikes a molecule that acts as a scattering center. The electric field can be decomposed into two components perpendicular to the direction of propagation. Let’s assume that the electromagnetic wave moves in -direction. Then one of the components swings along the -direction and the other along the -direction.

The component in -direction also makes the molecule oscillate along the -axis. Thus it emits radiation both in -direction and in -direction.

The component in -direction, on the other hand, makes the molecule vibrate along the -axis. It therefore emits radiation in -direction and in -direction.

Overall, therefore, the radiation in -direction is still not polarized, while it is in -and -direction.

Polarization Examples

Finally, we present you a short list of applications where the polarization of light is crucial. These include