The Student Room Group

polarisation - what am i missing here?

i'm sure i havent understood polarisation properly :frown:
an electromagnetic wave is an electric field and a magnetic field oscillating perpendicularly to each other and to the direction of propagation, right? and when you pass a wave through a polariser it only lets the components in one plane through it. so if the polariser is in the same plane as, say, the magnetic field, wouldn't you just get an electric field on the other side? my teacher explained it once but i dont get it :mad:
and another polarisation thing: when i did my work experience i worked with this laser thingy called a MOKE (magneto-optical kerr effect) and they explained it to me like this: the light is polarised in one direction, the magnetic field rotates the polarisation, then it passes through another filter perpendicular to the first. what i want to know is how the magnetic field rotates the plane of polarisation. if its going to be too complicated then i'm not bothered at the moment (too much revision) but it would be nice to know!
physicsgirlie
i'm sure i havent understood polarisation properly :frown:
an electromagnetic wave is an electric field and a magnetic field oscillating perpendicularly to each other and to the direction of propagation, right? and when you pass a wave through a polariser it only lets the components in one plane through it. so if the polariser is in the same plane as, say, the magnetic field, wouldn't you just get an electric field on the other side? my teacher explained it once but i dont get it :mad:
and another polarisation thing: when i did my work experience i worked with this laser thingy called a MOKE (magneto-optical kerr effect) and they explained it to me like this: the light is polarised in one direction, the magnetic field rotates the polarisation, then it passes through another filter perpendicular to the first. what i want to know is how the magnetic field rotates the plane of polarisation. if its going to be too complicated then i'm not bothered at the moment (too much revision) but it would be nice to know!

A polariser doesn't work like that. It's not like a big slit which saws off anything that doesn't fit through it. Instead it forces the Electric (and magnetic field) to be aligned in a particular direction with respect to it. Say you have a vertical polariser, it will only allow vertical E (and corresponding horizontal B) and not horizontal E (and corresponding vertical B).

As for the other question I don't actually know off the top of my head, unless it was circularly polarised light rather than plane polarised?
Reply 2
is the magnetic field you talk about in the second part of your question an external magnetic field? I.e. are you implying that they were passing a laser through an external magnetic field and this affected the polarization?
Reply 3
physicsgirlie
i'm sure i havent understood polarisation properly :frown:
an electromagnetic wave is an electric field and a magnetic field oscillating perpendicularly to each other and to the direction of propagation, right?

This is the simplest wave, yes. A plane, monochromatic, linearly polarised wave.

and when you pass a wave through a polariser it only lets the components in one plane through it. so if the polariser is in the same plane as, say, the magnetic field, wouldn't you just get an electric field on the other side? my teacher explained it once but i dont get it :mad:

No. A polarising material like Polaroid can be thought of as acting only on the electric field. Imagine a Polaroid sheet as a like lots of thin parallel wires all in the same direction - like a tennis net with all the horiztonal bits of string removed, for example. The wires all go from top to bottom in parallel if we have a horiztonal polariser.

When the electromagnetic wave comes along, the E-vectors that are parallel to the wires will induce voltages in the wires, and thus currents will flow under resistance that remove the energy from that component of the E-field in the wave. In a more complicated explanation, the vertical E-vector components see a great change in impedence, resulting in a lot of absorption and reflection, whilst the horiztonal E-vector components do not see much difference.

Hence, the only E-vectors component that 'gets through' is the one perpendicular to the vertical 'wires' i.e. the horizontal components. Hence, it is a horiztonal polariser.

and another polarisation thing: when i did my work experience i worked with this laser thingy called a MOKE (magneto-optical kerr effect) and they explained it to me like this: the light is polarised in one direction, the magnetic field rotates the polarisation, then it passes through another filter perpendicular to the first. what i want to know is how the magnetic field rotates the plane of polarisation. if its going to be too complicated then i'm not bothered at the moment (too much revision) but it would be nice to know!

This is the Faraday effect. Yes, it is quite complicated but interesting all the same. It works because the phase introduced in the clockwise circularly polarised wave is different to the phase introduced in the anti-clockwise circiularly polarised light.

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

I believe it has applications in lasers to do with mode-locking.
Reply 4
cheers!

it was in the context of measuring the magnetic moment of a particular sample at varying applied fields. condensed matter stuff i think...