Polarisation of Light: Causes & Effects

[Cheman]

Polarisation of light...

With light, the elctric and magnetic fields are perpendicular to the direction of wave travel - but my textbook says that these also exist in every plane, forming almost like a cylinder. This is what means it can be polarised. But what causes this? ie - what are the electrons doing to cause the elctric field to be in all directions?

Further more, what happens to the magnetic fields when light is polarised - does this remain perpendicular to the elctric field remaining?

Thanks in advance.

 

[James R]

The electric and magnetic fields are always at right angles to each other in an electromagnetic wave. By convention, the direction of polarisation of the light is taken to be the direction of the electric field vector.

At a single point in space, the electric field vector can only point in one direction at a time. If it maintains the same orientation along the wave, the wave is plane polarised. If the field vector rotates along the wave, then the wave is circularly (or elliptically) polarised.

 

[Claude Bile]

Light may also be unpolarised. Unpolarised light is the sum of many polarised waves, randomly oriented. Unpolarised light is characterised by the fact that when one decomposes the field into two orthogonal polarisations, the components are always equal. In practical terms, this means that only 50% of light will go through a polaroid, no matter the orientation of the polaroid.

Claude.

 

[SpaceTiger]

This is what means it can be polarised. But what causes this? ie - what are the electrons doing to cause the elctric field to be in all directions?

The type of polarization depends on how the electron is moving perpendicular to your line of sight. Motion toward or away from you will not induce an electromagnetic wave, but tangential motion will. If the electron is oscillating back and forth in a line, the light will be linearly polarized. If it's spinning in a circle, then it will give circularly polarized light. Finally, if there are many electrons and they're all moving in random directions, then the result will be unpolarized light.

Direct light from the sun, for example, is unpolarized because its emission is coming from charges undergoing random thermal motion in the photosphere. On the other hand, scattered light from around the sun will be linearly polarized because the scatterers will be oscillating in a plane perpendicular to the line of sight. Finally, magnetic fields are a common source of circularly polarized light.