Spark image


If you get hold of one end of a rubber rope, tie the other end to a post, stretch it and then send a series of pulses down the rope the vibration travels down the rope. Although each successive pulse may be sent in a different plane each pulse only vibrates in one direction.
This is exactly the same as a source of light. Each quantum emitted has vibrations in one plane but because you receive many millions of quanta per second from a light source it appears that the wave is vibrating in all directions.

A wave in which the plane of vibration is constantly changing is called an unpolarised wave.

However if the vibrations of a transverse wave are in one plane only then the wave is said to be plane polarised.

When light is plane-polarised the vibrations are made to occur in one plane only. Light is a transverse electromagnetic wave with the vibrations of an electric and a magnetic field occurring at right angles to each other and in any plane at right angles to the direction of travel of the light.

Polarisation is easily observed with the rubber rope experiment described above but it can also be shown with electromagnetic waves such as microwaves, TV, radio and light.

It is important to realise that transverse waves can be polarised while longitudinal waves cannot.

Effects of polarisation with light

Car windscreens
Polarisation by reflection - glare/shine off roads
Optical activity
Polarisation of scattered sunlight
Stresses in materials
Liquid crystal displays on a calculator
Lap top displays

Polariser and analyser

The effect of a polariser and an analyser is shown in the following diagrams (Figure 3).

Student investigation 1
Simulation of the polarisation of scattered light in the atmosphere. Fill a transparent rectangular plastic tank with water and add a little milk to it (a few millilitres per litre should be sufficient). Shine a bright beam of light through the mixture and observe the polarisation at different points around the tank using a piece of polaroid and an LDR connected to a meter.

Student Investigation 2
Measure the amount of light transmitted through a pair of sunglasses when the lenses are placed over each other and rotated. Also see if they remove the reflection from a shiny surface such as a hot road, a sheet of glass, a wet board or the surface of a swimming pool.

Student Investigation 3
Study the polarisation of TV and radio signals in your area.

The phenomenon of the polarisation of light was known to Newton and was inexplicable in terms of longitudinal waves. Later theory showed that light is a transverse wave motion and in fact polarisation is very good evidence for this type of wave motion. Polarisation was first recorded by Bartholinus in 1669 when he noticed the double refraction in Iceland spar, a crystalline form of calcium carbonate (double refraction is discussed later in this chapter).

In 1808 Malus found that if he looked through two tourmaline crystals and then rotated one through 90o the light was cut out. He also looked through a tourmaline crystal at the windows of the palace in Luxembourg which were reflecting the setting Sun and found that this reflected light could be cut off by rotating the crystal. This showed that the reflected light was polarised.

As you know, waves can be either transverse or longitudinal in nature. If the vibrations of a transverse wave are in one plane only then that wave is said to be plane-polarised. (Longitudinal waves cannot be plane-polarised.)
The human eye cannot distinguish between polarised and non-polarised light; what we actually register is the intensity of light. Some materials can be used to detect polarisation, however, by allowing light to pass through them only if the vibrations are in a particular plane, called the plane of polarisation of the material.

One such material is herapathite, a sulphate of iodoquinine discovered in 1852. The plastic known as Polaroid, developed by Land in 1932, is a sheet of nitrocellulose with millions of herapathite crystals embedded in it, and is often used in the laboratory to polarise light.

© Keith Gibbs 2011