After working through this chapter, you should be able to:
Explain the difference between polarised and unpolarised light
Explain the process of polarisation by transmission
Explain the process of polarisation by refraction
Explain the process of polarisation by reflection
Explain the process of polarisation by scattering
We’ve already discussed a lot about light and how it can be emitted from a source in all directions (e.g. like the sun), and more specifically we’ve talked about how light can be expressed as waves , travelling outwards from the source. This chapter will focus on how we can alter the orientation of those waves, through a process called polarisation .
Some light revision
(Pun intended.) When light is emitted from a light source, it can be described in terms of its wavefronts, light rays or light waves. To understand polarisation, we need to focus on light as a wave .
Imagine a single light wave is being emitted from a light source (in this case, possibly a laser), as shown in Fig. 14.1 A. In this example, the light is oscillating vertically, which means its electric field (or electric vector ) is also vertically oriented. We learned in chapter 1 that visible light is part of the electromagnetic spectrum, but we never specifically discussed that it is technically an oscillating electric and magnetic field (both of which are oriented perpendicular to one another). The oscillating electric field that forms the light has the potential to be able to affect electrons in other materials by causing them to start oscillating too. This oscillation in other electrons forms part of the basic explanation of how polarisation works, but we don’t need to get too bogged down in this right now.
If we now go back to thinking about our single light source, I think we can agree that it’s relatively easy to imagine light as a single waveform with a vertically oriented electric field (and much easier to draw that way too); however, most light sources (the sun, LED bulbs) produce light that contains vibrations in all possible meridians (otherwise known as orientations/planes), as shown in Fig. 14.1 B (which unfortunately is much harder to draw and visualise). In this case, the light is formed of many orientations of light, so the electric field changes orientation randomly over time.
When light vibrates in all directions like this (see Fig. 14.1 B), it’s considered to be unpolarised , because it has more than one orientation of vibration (and a slightly unpredictable electric field). However, it is possible to take unpolarised light and reduce it to a beam of light comprising vibrations that occur (mostly) within a single meridian, which we would then describe as polarised light. This process of transforming the unpolarised light into a polarised state is called polarisation , and polarisation can occur through a number of different methods, including: (1) transmission , (2) reflection , (3) refraction and (4) scattering . First, however, let’s discuss types of polarisation.
Types of polarisation
For light to become polarised, we need to limit the orientations (or confine the direction of the electric field vector). This can be done linearly, circularly or elliptically.
Linear polarisation occurs if the electric field of the light (orientation of the wave) is restricted to a single plane/orientation ( Fig. 14.2 A). This type of polarisation will be discussed most often within this chapter as it’s the ‘easiest’ to understand. In contrast to this, circular polarisation comprises two linearly polarised (perpendicular) waves that possess a phase difference of 90° whilst possessing the same amplitude ( Fig. 14.2 B). In this case, the electric field produced by these waves will rotate as the waves move forward, in a circular shape. This can be clockwise or anticlockwise and is usually referred to as right-hand or left-hand circular polarisation, depending on which of the waves is ‘ahead’ (in terms of phase) of the other. If two linearly polarised (perpendicular) waves possess different amplitudes or a phase difference that varies from 90°, then this produces elliptical polarisation ( Fig. 14.2 C).
Now polarised light can also be classified as either p-polarised or s-polarised depending on how it is polarised relative to what’s referred to as the plane of incidence . The plane of incidence can be thought of as a flat, completely imaginary surface on which the incident (and reflected) light exists – so, often, the plane of incidence is drawn perpendicular to the surface ( Fig. 14.3 ). P-polarised light describes light that possesses an electric field that is parallel to the plane of incidence (‘p’ for parallel), whereas s-polarised light describes light with an electric field that is perpendicular to the plane of incidence (‘s’ for s’not parallel at all . . . or, technically, s for ‘senkrecht’, which is German for perpendicular). So in Fig. 14.3 if the reflected light was parallel to the plane of incidence (vertical) then it would be p-polarised, and if it was perpendicular to the plane of incidence (horizontal), it would be s-polarised.
Polarisation by transmission
One of the most common man-made methods of polarising light is to use a material as a polariser (a system that can turn unpolarised light into a polarised state). These materials are specially designed to only transmit waves vibrating in a single direction (much like in the example in Fig. 14.4 ). Now, because this method of polarisation will transmit vibrations of light parallel to the transmission axis (or polarisation axis) of the polariser, this method is often called polarisation by transmission . In Fig. 14.4 , the polarising filter shown has a vertical transmission axis, so only vibrations oriented vertically will be transmitted through. All other vibration directions will be stopped by the filter, particularly those oriented perpendicularly to the transmission axis. An easy way to remember this is to liken it to the ‘picket fence’ analogy. This analogy assumes that a dog (for example) with a stick in its mouth will only be able to pass through a gap in a picket fence if the stick is vertical (meaning the dog will need to turn its head). Holding the stick horizontally will not work – just like with this example of polarisation!
This also means that if you put two perpendicularly aligned polarisers one after another, the light would become polarised as it transmits through the first polariser, but it would then get stopped by the second polariser. These are called crossed polarisers ( Fig. 14.5 ).
Importantly, as these polarisers are technically stopping quite a good amount of light from passing through to the other side, there is a reduction in brightness (∼50%) of the light after transmission. This helps form part of the explanation as to why polarising filters make great lenses for sunglasses (but it isn’t the whole story). The benefit of having polarised lenses is discussed towards the end of this chapter.
Polarisation by reflection
Another method of polarising light can be to reflect it off a surface; in proper scientific terms this is due to the relationship between the electric field of the light relative to the plane of incidence upon the surface. Essentially, if the electric vector is perpendicular to the plane of incidence, then the light will be polarised in the same orientation as the electric vector. In a simple example, if we have incident light upon a horizontal surface (e.g. a lake), then the plane of incidence will be vertical (as shown in Fig. 14.6 ). This means that the light will ultimately end up being polarised to be parallel to the surface of the road (also horizontal), which will be perpendicular to the plane of incidence, making it s-polarised.