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Polarisation

Polarisation may have different meanings, but here the focus is on the polarisation of waves. Waves are disturbances that propagate in time and space. When considering their spatial properties, we may find a periodic behaviour if some patterns are repeated, or a stationary behaviour if some points are in a constant vibrational/non-vibrational state.Suppose we are holding one end of a…

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# Polarisation

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Polarisation may have different meanings, but here the focus is on the polarisation of waves. Waves are disturbances that propagate in time and space. When considering their spatial properties, we may find a periodic behaviour if some patterns are repeated, or a stationary behaviour if some points are in a constant vibrational/non-vibrational state.

Suppose we are holding one end of a very long spring whose other is held by another person. If we repeatedly pull it up and down, a wave appears whose displacement (up and down) is perpendicular to the direction of movement of the disturbance (forward). This is called a transverse wave.

On the other hand, if we climb some stairs, hold the spring vertically and pull it down, the spring will bounce upwards and oscillate. In this case, the displacement of the spring (up and down) is in the same direction as the disturbance (up and down). This is called a longitudinal wave.

If we go back to holding the spring on the same floor as the other person and start drawing big circles with the end of the spring, a disturbance will travel forward while displacing the string in a circular way.

Bearing all this in mind, we can define the polarisation of a wave as the geometrical orientation of the oscillations. It is also clear that it does not make sense to define the polarisation of a longitudinal wave since it is already specified by the direction of its propagation.

## Types of polarisation and their properties

Wave polarisation can come in several types, such as linear, circular, or elliptical polarisation. Each type has its own properties and can be found in different settings.

### Linear polarisation

Linear polarisation is the confinement of the transverse oscillations of a wave to a plane containing the wave’s trajectory.

The above is the case for electromagnetic waves. In fact, in this case, since we have both electric and magnetic waves, there are two planes on which each of the signals is confined. However, as we shall see, this situation is an ideal one. When there are no interferences, waves propagate in the vacuum.

Figure 1. Linearly polarised wave. Source: Wikimedia Commons (Public domain).

### Circular polarisation

Circular polarisation is the orientation of the oscillations of a wave such that their projection on a plane that is perpendicular to the direction of propagation draws a circular rotation.

This definition allows us to define linear polarisation in a similar manner by stating that a wave is linearly polarised when the projections of its oscillations on a plane perpendicular to the direction of propagation draw a straight line.

Figure 2.: A circularly polarised wave.

### Other types of polarisation

In general, both linear and circular polarisation are extremely rare cases. Usually, by using the projection of the oscillations on a perpendicular plane, we find that rotating vectors can draw many arbitrary shapes defining new kinds of polarisation. For instance, we find that some waves have an elliptical polarisation, but we could also find ‘star-like’ polarised waves.

Figure 3. Elliptical polarisation. Source: Wikimedia Commons (Public domain).

## Applications of polarisation

The polarisation of waves can be manipulated to serve different purposes, using devices known as polarisers. They have a wide variety of applications and can be found in everyday objects like sunglasses or photography cameras.

### Polarisers

The light we get from the sun is unpolarised. What does this mean? We know that waves carry an oscillation and, hence, must have a direction. Unpolarised means that all the waves we receive carry different, non-correlated polarisations (sometimes called randomly polarised). This amounts to a chaotic distribution of oscillations from which we cannot extract a distinct polarisation. Polarisers, however, can turn unpolarised light into polarised light.

Figure 4. Linear polariser turning unpolarised light into linear polarised light. Source: Fffred~commonswiki, Wikimedia Commons (CC BY-SA 3.0).

These devices operate by dimming the intensity of light in every direction except the ones in which we want the wave to oscillate. A good example of this is sunglasses, which have a polariser whose function is to dim light in certain directions, so the amount of light we are receiving is reduced. It is timely to remember that waves carry energy, and if part of the light is dimmed or neutralised, the amount of energy also is proportionally reduced. Usually, since polarisers are not perfect, the dimming is not perfect either, and we still perceive a fraction of the lights intensity.

### Polaroid photography and radio signals

Polaroid cameras also have integrated polarisers that intensify certain colours by dimming others and composing the image afterwards. These cameras allow us to take photographs on bright days since their filters act in the same way as sunglasses. This also applies to light reflected on surfaces, which turns out to be strongly polarised and may be dimmed by a polarising filter.

Another application of polarisers involves the emission and reception of radio signals. Returning to the concept of energy carried by a wave, since antennae need to interpret certain signals to extract information from them, a polarised wave will allow extracting this information in a more efficient way. That is why the emission of signals is done with polarisers.

## Key takeaways

• Waves are disturbances that have a geometrical orientation known as call polarisation.

• The best-known examples of polarisation are linear and circular polarisation, but most cases are more complex.

• Polarisers are devices that allow generating a certain polarisation of light waves.

• Many everyday phenomena, such as the emission of radio signals or some photography mechanisms, make use of polarisers.

Polarisation is the geometrical orientation of the disturbances generated by waves.

The polarisation of light is the geometrical orientation featured by electromagnetic waves.

Polarised lenses are lenses carrying a polariser. Their purpose is to dim a fraction of the incoming light.

By using a linear polariser, we achieve a linearly polarised light wave. Afterwards, we can use another polariser with two orthogonal directions such that one of them has a retarding system. This way, by tuning the delay and the amplitude dimming, one can achieve circular polarisation.

## Polarisation Quiz - Teste dein Wissen

Question

Only transverse waves have a polarisation.

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All transverse waves can be polarised.

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There are many types of polarisation, such as linear, circular, elliptical ….

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Question

Polarised lenses dim light coming from different sources, but not totally.

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The light from the sun is randomly polarised.

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Do Polaroid cameras work well in bright scenarios?

Yes, they do since they carry polarisers that allow to dim excessive light input.

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Question

Does it make sense to define a polarisation for longitudinal waves?

No, it does not make sense since the direction of oscillation can only be one.

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Question

Is unpolarised light useful for information transmission?

No, unpolarised light (or randomly polarised light) consists of many different waves with polarisations that do not allow us to extract information due to the lack of a pattern.

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Are radio waves on earth polarised?

Yes, they are polarised since this ensures efficient transmission of information.

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Question

What is the polarisation of a wave?

The polarisation of a wave is the geometrical orientation of the oscillations of the wave.

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Question

Do only periodic waves have a polarisation?

No, all transverse waves have a polarisation since they oscillate in certain directions. A periodic behaviour is not necessary.

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Question

What would happen if we used two linear polarisers, one after the other, in the same direction (starting from randomly polarised light)?

After the first one, light would be linearly polarised. Since the second one is in the same direction, the light will go on being polarised in that direction.

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Question

What would happen if we used two linear polarisers, one after the other, in perpendicular directions (starting from randomly polarised light)?

After the first one, light would be linearly polarised. However, when reaching the second one, the light would find its component perpendicular to the second polariser dimmed. Since they are totally perpendicular, no light would go through.

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Question

What would happen if we used a circular polariser and then a linear polariser (starting from linearly polarised light)?

After the first one, the light would have a circular polarisation. However, the second one then again enforces a linear polarisation.

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Question

What would happen if we used a linear polariser and then a circular polariser (starting from randomly polarised light)?

After  the first one, the light would have a linear polarisation. After the second one, the light would then have circular polarisation.

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