16 Polarisation

We have referred once or twice to polarisation, but have yet to examine it in detail.

The fact is that light can be polarised. The simplest way to see this is to take an old pair of polarised sunglasses and remove one or both lenses. When you place one lens on top of the other, and sight through both, what you see depends on the relative orientation of the pair. As you rotate one lens, there are two positions, 180º apart, where light fails to get through.

From this it is clear that light, at least after passing through the first lens, has an orientation and that this is at right angles to the direction that it is travelling. This allows us to identify some light as polarised, and to ascribe to it an angle of polarisation, such as ‘vertical’, ‘horizontal’, or ‘at 45 degrees’.

Normal, unpolarised light, when passing through certain material, is split up into separate rays, distinguished by polarisation. This has long been known for the common material calcite where it is termed birefringence. This is a property of those crystalline materials where the crystal structure is anisotropic, meaning in this case a different structural pattern when viewed in different directions.

Polarised light, from either source, has the property that signals of opposite polarisations will not create interference patterns in the normal manner of light and other waves. It is much easier to see how this can occur with transverse waves as opposed to longitudinal ones, and this was an important consideration in leading Fresnel in 1821 to conclude that light is a transverse wave.

A modern application of polarisation is in the transmission of television broadcasts. In order to minimise distortion of the signal in transit, as would happen, for example, if it passed through calcite, signals are transmitted in polarised form. This has an additional benefit. Some areas can receive signals from different transmitters, and this feature reduces the interference between these on reception. This is why the side prongs on TV aerials, which act as wave guides, have a horizontal or vertical orientation, and why it is important to install them with the correct one.

There is, of course, no ‘transverse wave’ in modern theory, as any kind of wave requires a medium of propagation and that has been rejected, albeit on the basis of flawed reasoning. Instead, modern physics talks of a wave without medium or substance that has nevertheless transverse variations.

This conflates the fact of ‘transverse variation’ with the idea of a ‘transverse wave’, and it is fairly easy to see that this is inappropriate. Let us start with the thought experiment of dropping a stone in a pond. We get an expanding circular wavefront that is smooth. This is a transverse wave as the up and down motion of the water, the wave motion, is transverse to the direction of travel. Now drop two stones simultaneously; from some directions the wavefront is more complicated, and has variation in a direction that is not the direction of travel, but neither is it in the direction of oscillation.

Now imagine it with pressure waves. We can ‘see’ that two sound sources can create transverse variation despite the fact that the waves are clearly longitudinal. We may even observe this phenomenon at times by noting a difference in sound quality from stereo speakers when moving the head from side to side.

Nor does this phenomenon occur only with multiple sources. We can create similarly transverse effects in water and air by oscillating a single sound source from side to side. Longitudinal waves are capable of transverse variation, and it is disingenuous of physicists to imply that they are not. The construction of polarising lenses involves the inclusion of elongated molecules with different optical properties from the embedding material, and this appears designed to create precisely the corrugations in the wave pattern of a longitudinal wave that are required.

Comments are closed.