Polarisation can mean a lot of things, but here we're talking about waves. Waves are like ripples that move through space and time. When we look at how they move through space, we can see that they might repeat in a pattern or stay in one spot.

Imagine holding a long spring with someone else. If you pull it up and down over and over, a wave will form that moves up and down. This is called a transverse wave. But if you hold the spring vertically and pull it down, it will bounce up and down in the same direction as the disturbance. This is called a longitudinal wave.

If you draw big circles with the spring while holding it on the same floor as the other person, the wave will travel forward while making a circular shape.

So, polarisation is just a fancy word for the way that the wave moves. It's like the direction the wave is facing. But if the wave is already moving in the same direction as it's facing, we don't need to talk about polarisation.

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 when a wave's up-and-down movements are limited to one plane along the direction it travels. This happens with electromagnetic waves, which have both electric and magnetic components. In this case, each signal is limited to two planes. But this is just in an ideal situation with no interference. When waves travel through a vacuum, they can move freely without interference.

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Circular polarisation

Circular polarisation is when a wave's up-and-down movements rotate in a circle perpendicular to the direction it travels. This means that if you looked at the wave from the side, it would look like a spiral. In contrast, linear polarisation is when the wave's up-and-down movements are limited to a straight line perpendicular to the direction it travels. So, if you looked at the wave from the side, it would look like a flat line.

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Other types of polarisation

Linear and circular polarisations are not very common in reality. When we look at the projection of a wave's up-and-down movements on a plane perpendicular to its direction of travel, we often find that the wave's vector can draw many different shapes. These shapes define new types of polarisation that are not linear or circular. For example, some waves have an elliptical polarisation, while others may have a more complex 'star-like' polarisation.

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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

When it comes to the light we receive from the sun, it is considered to be unpolarised. This means that the waves of light that make up sunlight are not all aligned in the same direction. Instead, they are randomly polarised, which means that their oscillations do not have a consistent direction. The result is a chaotic distribution of oscillations that does not give us a distinct polarisation to work with. However, we can use polarisers to turn unpolarised light into polarised light by filtering out waves that are not aligned with a specific direction.

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Polarisers work by reducing the intensity of light in every direction except for those in which we want the wave to oscillate. This is achieved by using a material that filters out waves that are not aligned with a specific direction. For example, sunglasses have a polariser that reduces the amount of light that reaches our eyes by filtering out waves that are not aligned with the of the polariser. It is important to note that waves carry energy, and when part of the light is filtered out or dimmed, the amount of energy is also reduced proportionally. However, since polarisers are not perfect, the reduction in intensity is not always complete, and we still perceive a fraction of the light's original intensity.

Polaroid photography and radio signals

Polaroid cameras use integrated polarisers to enhance certain colours by dimming others and then composing the image. This feature makes it possible to take photographs on bright days, as the camera's filters act in the same way as polarised sunglasses. Similarly, light reflected off of surfaces can also be strongly polarised and may be dimmed by a polarising filter. Another application of polarisers involves the emission and reception of radio signals. Since antennae need to interpret certain signals to extract information from them, a polarised wave can allow for more efficient extraction of this information. That's why polarisers are used when emitting signals.

To summarize, waves are disturbances that have a geometrical orientation known as polarisation. The most common types of polarisation are linear and circular polarisation, but most cases are more complex. Polarisers are devices that allow for the generation of a certain polarisation of light waves. Many everyday phenomena, such as the emission of radio signals or certain photography mechanisms, make use of polarisers to enhance their functionality.

Polarisation

What is polarisation?

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

What is the polarisation of light?

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

What are polarised lenses?

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

How do you produce circularly polarised 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.

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