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 light’s 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.
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