What is polarisation?

Question

Hey everyone,

I'm doing an electromagnetism course, just stuck on what the term polarised means..

and what does it mean by 'linearly polarised?'

the book says:

"A wave propagates in the xz plane at 30degrees (to the x axis) and is linearly polarised along y."

If anyone could clear that up for me, it would be most appreciated, thanks!

Answer 1

In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field.

http://en.wikipedia.org/wiki/Polarized

i hope this helps!

Answer 2

Right,

In the first link, look at the first picture, showing linear polarization.

Notice that as the wave travels, its electric-field oscillation remains in one orientation.     A certain number of degrees.

Light commonly found all around you, is reflected, and scattered light from many sources.   It consists of a random mixture of orientations of the electric field.

The  transverse electromagnetic wave is shown with only one orientation.   Where all other orientations of light have been filtered away.     (un-realistic).

After common un-polarized light with all orientations have passed through a polarized filter,  only the one orientation dominates, and could be said to be polarized.

Die-electric materials can be used to polarize light by reflection.  Some of the light is reflected, and is polarized.

( See the second link on polarization of light, you have to scroll down a bit).  This link also shows circular polarization, animation so you can see the difference between linear and circular.

Common filters are not that good.

Actually when light passes through a common polarizing filter,  it does not cut off all other polarizations perfectly as shown in the photograph.  The attenuation depends on the angle at which any transverse wave passes through the filter.

A series of filters could be used to progressively attenuate away the orientations of light that are not of the desired polarization.

Passing light through crystaline structures can cause the polarization of the incomming light to change orientation by a certain number of degrees.   Organic compounds can be crystalized, and identified by the degree at which light is rotated by passing light through them.  Some rotate to the right, and some rotate to the left.

How can you make a polarizer?  Easy,  

You can make a series of finely placed bars on a film, like closely placed bars in a cage.   The spacing between the bars must be close to the wavelength of the light to be polarized.  EM waves of the correct orientation can pass through with miminal interference, but will be partially or totally blocked if the orientation is not optimal.

Polarizers for sun glasses are made in this way, using photographic metal deposition of the fine lines.

If you have two filters turned 90 degrees to one-another, then no light will pass.

Polarized sun glasses use a polarizer to advantage, by filtering out glare, particularly from water.  Light from the sun is unpolarized, and if reflected from metal(good conductor) or rough rocks remains unpolarized.  But if reflected off of some smooth dielectric substances such as water, much of the reflected light is polarized.   The filter in the sun glasses removes much of this due to its orientation to the filter.

Liquid crystal displays on common electronic devices use this property to display data.  A mirror in the back is used to reflect the light back out. Liquid crystal sandwiched in-between can rotate light when placed across an electric field.   The glass substrate is a polarized filter, so when light enters, it normally passes through the filter, crystal, and bounces off the mirror back to you. But when the electric field is present, the light is rotated, and when it reflects back, it is of the wrong orientation to pass back through the polarizer, and appears to be black.  You can verify this, by getting a good pair of polarized sun glasses.   If you look at an LCD display straight on, it will be black.  If you rotate the display, it will appear mirrored as usual.   This is because the LCD polarizer filters out most of the light in the wrong orientation, and the polarizer in the sun glasses can filter out the rest.

Answer 3

Polarization is a property of transverse waves which describes the orientation of the oscillations in the plane perpendicular to the wave's direction of travel. In electrodynamics, polarization characterizes electromagnetic waves, such as light, by specifying the direction of the wave's electric field.