Polarisation

Imagine a wave travelling in a direction z.
x and y are two directions perpendicular to this direction.
(The magnetic field is not shown. It is always perpendicular to and proportional in magnitude to the electric field.)

The wave shown in the diagram above is plane polarised. That means that its electric field has a component in just one direction only, in this case the y direction.

The waves from most sources are not polarised. The electric field has a component in both the x and y planes. If the two components are in phase (though they aren't always) then the situation below results.

Later we will us a photon model to describe light. It may be useful to think of all the individual photons having electric fields randomly orientated to the direction of propagation (travel).

When unpolarised light passes through a polarising filter it emerges polarised. This is because the filter absorbs one of the components and so just one of the two is transmitted (gets through). Such a filter is made up of long chain molecules which absorb the energy of the wave just like an aerial absorbs the energy of a radio wave. It will only absorb the electric field component of the wave which is parallel to the molecules so the component which is perpendicular will get through.

If you used two polarising filters with molecules at right angles to each other, i.e. crossed polaroids, then nothing will get through. Rotate one of them 90 degrees and light will get through. Some of the light will be absorbed as one of the components of the E field has been absorbed.