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.
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.