Polarisation

The phenomenon: vibrations in a plane

Radio waves can be both emitted and detected with long stick-like electrically conducting metal aerials. It is not too much of a stretch for the imagination to think of the electrically charged particles moving up and down the emitting aerials, and so being the source for the vibrations that spread to all the surrounding detectors. So you can picture the transverse nature of the vibrations: the electrons move up and down and the influence travels radially away from the aerial.

These vibrations are at right angles to the direction of travel between source and detector, and they are even more constrained than that – the electric component is moving up and down only.

That is why you can significantly alter the effectiveness of the detection by varying the angle at which the small domestic aerial is pointing. The vibrations may pass it by because they are not in the correct plane. Rotate the aerial far enough and it will be at right angles to the vibrations from the source, and you'll get no signal at all. The vibrations are polarised.

Exploiting polarisation by taking care with orientation

Remembering that light is one of a family of electromagnetic waves, you might set out to look for similar effects in other members of the family, even where the source is not obviously vibrating at right angles to the direction in which the vibrations are radiated. Such a quest might find you fishing by a lakeside, at sunset, trying to see into the water. You'll find it hard due to the glare from the surface. But as every complete angler knows, slipping on a pair of polarised glasses helps greatly. These block nearly all the vibrations in one direction and pass nearly all the vibrations at right angles to this direction. The beam reflected off the water is mostly vibrating from side to side, parallel with the surface, while still transverse to the direction of travel.

The beam reflected off the fish, enabling us to see them, is not constrained to one plane as it has not bounced off the flattish surface of the water, which acts like the source aerial in the first example. In that example the source was polarised; in this example the beam becomes polarised by reflection.

A third way of polarising a beam is by filtering. That is what polarising sunglasses do – they remove all the vibrations except those in one plane. So the beam reflected from the fish in the previous example is less intense than it would be without the polarising filter. However, the effect on the polarised beam is much greater, as the polarising filter is arranged to be vertical and so at right angles to the plane of vibrations of the incoming beam. Much of the glare that the sunglasses are designed to eliminate comes about from reflections off nearly horizontal surfaces, such as water and snow.

Polarisation is most easily understood as if there were two beams of vibrations. But in unpolarised light there are are vibrations in all possible planes at right angles to the direction of motion. So there is a bit of a challenge. But you can get a vibration in any direction you choose by combining fractions of the up and down vibration with the left and right vibration. In this way you can compose a vibration of any amplitude, in any direction. So it's a useful model to decompose these multiple vibrations to two planes at right angles to each other, either by adding the appropriate components from each of these planes (synthesis) or by resolving the desired vibration into these two components (analysis).

Polarisation has uses beyond increasing the density of radio broadcasting. Signals from adjacent stations can be at right angles and so not affect each other, even if they are broadcasting at the same frequency, or seeing more clearly over snow or water, or maybe even seeing through shop windows, but you'd expect to have to turn your head sideways to the ground for this to work, wouldn't you?

The sunlight is polarised, and bees use this to navigate. Photographers exploit this, using polarising filters to accentuate cloudscapes. Some molecules rotate the plane of polarisation, so chemists exploit this to measure concentrations; more molecules are assumed to lead to more polarising.

LCD screens such as those on mobile phones, calculators and GPS systems use polarised light to switch on and off different parts of the screen – so making it hard to read these devices with polarising sunglasses on. Try using these gizmos with polarising sunglasses on to check that this is so.

Polarised light arises as one plane of vibration is selected – by reflection, by filtering, or simply because the source is designed that way. Remember the radio transmitter?

One can even obtain circularly polarised light by having the two components out of step.

A further polarising filter can be used as a detector to show the plane of polarisation. The filter will permit the largest transmission intensity when aligned with the incoming vibrations. The minimum is permitted when it is at right angles to those vibrations.