# Polarisation and the EM field

1. Sep 30, 2009

### ananthu

1. The problem statement, all variables and given/known data
"Polarisation" is defined as the cofinement of the vibrations of the wave in only one plane and the removal of the vibrations in the other perpendicualar plane of the electromagnetic wave. But the e.m. wave is defined as a wave in which the electic vetors are restricted in one plane only where as the magnetic vector is vibrating in the other plane, i.e,. the em wave is itself is polasrised only as per above definition.
In that case what does further polaristion of the wave mean? Removing the vibrations in the other perpendicular plane means removing the magnetic vector? Also if we assume that the electric field has components in all possible directions and out of these vibrations, those in one plane are removed and those in a plane perpendicular to it is retained, will it lead to the conclusion that the electric field vector has components parallel to the magnetic field vector also i.e.in the x-z plane ( assume the electric field is vibrating in the x-y plane and the magnetic field in the x-z plane, x-axis being the direction of propagation)?
If that is so, will it not violate the very definition of the em wave which says that these are the waves in which electric filed, magnetic field and the direction of propagation all are mutually perpendicular to each other? Will any body kindly clarify these points?

2. Relevant equations

3. The attempt at a solution

2. Sep 30, 2009

### tiny-tim

Hi ananthu!

From http://en.wikipedia.org/wiki/Polarised_light" [Broken] …
In other words, the magnetic field is still there, but its plane of polarisation is perpendicular to the direction specified.

You mustn't think of a polariser as like a grating which will only let one plane through …

if you do, then you have to say that it lets the perpendicular plane through for the magnetic field, which rather destroys the analogy.

(and there isn't anything special about either the electric or magnetic components anyway … essentially, they're two arbitrary complementary components of the whole 6-parameter electromagnetic field, just as "our" time and "our" space are two arbitrary complementary components of space-time )

Last edited by a moderator: May 4, 2017
3. Oct 1, 2009

### ananthu

Re: polarisation

Thank you for your reply. But my question is that whether the electric field itslef is already polarised or not in an em wave? The definition misleads. It says that in an e.m. wave, the electric vector is restricted in only one plane. So when you rotate a crystal in the path of an unpolarised light, no light should come out of the crystal at all when its optic axis is held perpendicular to a previously allowed plane. But a polariser allows light vibrations whichever are parallel to its axis in any plane to pass through, irrespective of its orientation. Only the second crystal placed in the path of a plane polarised light cuts the vibrations when it is rotated.