The Naked Scientists

The Naked Scientists Forum

Author Topic: Polarisation of visible light.  (Read 3667 times)

Offline RTCPhysics

  • Full Member
  • ***
  • Posts: 66
  • Thanked: 4 times
    • View Profile
Polarisation of visible light.
« on: 23/10/2014 12:39:01 »
When visible light is polarised into a single plane, which of its orthogonal electric and magnetic components is eliminated?


 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1880
  • Thanked: 145 times
    • View Profile
Re: Polarisation of visible light.
« Reply #1 on: 23/10/2014 13:06:48 »
Neither. Both are limited to one plane, but these planes are still orthogonal. In normal (non-polarized) light, the electric fields oscillate in every direction orthogonal to the direction of propagation, and each of these electric field oscillations has a corresponding orthogonal magnetic field oscillation. A polarizer just removes light that does not have a particular orientation.
 

Offline RTCPhysics

  • Full Member
  • ***
  • Posts: 66
  • Thanked: 4 times
    • View Profile
Re: Polarisation of visible light.
« Reply #2 on: 23/10/2014 17:59:42 »
Thanks for your explanation. I've always understood that the electric sine wave and the magnetic sine wave, though peaking at the same time, vibrate in different planes. One is in the XY plane and the other in the XZ plane. There are any number of diagrams published showing this. Hence my question.

If I've understood your reply correctly, both vibrate in the XY plane, either side of the direction of travel along the X axis. This certainly explains polarisation.

But why are so many diagrams incorrect? Have I still misunderstood your explanation?


 


 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2773
  • Thanked: 38 times
    • View Profile
Re: Polarisation of visible light.
« Reply #3 on: 23/10/2014 18:23:32 »
Quote from: RTCPhysics
But why are so many diagrams incorrect? Have I still misunderstood your explanation?
Hi RTCPhysics. Welcome to the forum! :)

What diagrams are you referring to? Are you aware of the fact that there are various kinds of polarization? They are:

1) Circular polarization
2) Elliptical polarization
3) Linear polarization

Could it be possible that you're confusing the diagrams?  See the ones at:

http://www.photonics.com/Article.aspx?AID=35808
http://www.microscopyu.com/articles/polarized/polarizedlightintro.html
http://en.wikipedia.org/wiki/Polarization_(waves)
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html
http://www.optics4kids.org/home/content/other-resources/articles/polarized-light/

These are all different sources whose diagrams all say the same thing depending on the kind of polarization.
 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1880
  • Thanked: 145 times
    • View Profile
Re: Polarisation of visible light.
« Reply #4 on: 23/10/2014 18:49:34 »
Thanks for your explanation. I've always understood that the electric sine wave and the magnetic sine wave, though peaking at the same time, vibrate in different planes. One is in the XY plane and the other in the XZ plane. There are any number of diagrams published showing this. Hence my question.

If I've understood your reply correctly, both vibrate in the XY plane, either side of the direction of travel along the X axis. This certainly explains polarisation.

But why are so many diagrams incorrect? Have I still misunderstood your explanation?

The magnetic wave is always perpendicular to the electric wave, but there is a phase shift--the magnetic wave peaks when the electric field is changing fastest, not when the electric field peaks.

Non-polarized light traveling in the x direction has contributions of electric waves in the z direction and the y direction and everywhere in between. (\ | / etc.). z-polarized light traveling in the x direction will only have electric field oscillations in the z direction (and magnetic oscillations in the y direction).
 

Offline RTCPhysics

  • Full Member
  • ***
  • Posts: 66
  • Thanked: 4 times
    • View Profile
Re: Polarisation of visible light.
« Reply #5 on: 24/10/2014 19:37:15 »

Thanks for your replies. Hi, PmbPhy. I have only been considering Linear polarisation. The now obvious insight that you've given me (thanks) is that a single photon of light is, by definition, polarised. Four of the references you gave me, show the photon as an electric vector and a magnetic vector in phase, but functioning at right angles to each other and to their direction of travel. So if this photon of visible light is incident upon a polariser with its electric vector orientated in such a way that it will pass through the polarising material, how does the magnetic vector also pass through, being at right angles to the polarising axis and hence presumably blocked.

I can only see three possibilities. Firstly that the polarising material does not interact with a magnetic field and hence lets the photon through. Secondly, that a photon can only pass through a polariser, if its magnetic vector is zero. Thirdly, that a photon does not have a magnetic vector.

Hi, chiralSPO. Thanks for your post too. I picked up upon your statement that the electric and magnetic vectors of a photon are not in phase, but a quarter wavelength out of phase with each other. It made sense to me that the magnetic vector followed the electric vector. However, the standard diagrams I'd seen for the photon showed them to be in phase, peaking at the same time. Are these illustrative diagrams for linear polarisation misleading? (See references 1 to 4 of PmbPhy post.)



     
 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1880
  • Thanked: 145 times
    • View Profile
Re: Polarisation of visible light.
« Reply #6 on: 24/10/2014 22:50:04 »
Apparently I am mistaken about the phase shift. My bad! They are in phase. This excerpt is from the wikipedia page on electromagnetic radiation.

"Electromagnetic radiation is a transverse wave, meaning that the oscillations of the waves are perpendicular to the direction of energy transfer and travel. The electric and magnetic parts of the field stand in a fixed ratio of strengths in order to satisfy the two Maxwell equations that specify how one is produced from the other. These E and B fields are also in phase, with both reaching maxima and minima at the same points in space (see illustrations). A common misconception is that the E and B fields in electromagnetic radiation are out of phase because a change in one produces the other, and this would produce a phase difference between them as sinusoidal functions (as indeed happens in electromagnetic induction, and in the near-field close to antennas). However, in the far-field EM radiation which is described by the two source-free Maxwell curl operator equations, a more correct description is that a time-change in one type of field is proportional to a space-change in the other. These derivatives require that the E and B fields in EMR are in-phase (see math section below)."
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4131
  • Thanked: 249 times
    • View Profile
Re: Polarisation of visible light.
« Reply #7 on: 24/10/2014 23:01:41 »
Quote
which of its orthogonal electric and magnetic components is eliminated?
Many types of commercial polarising filters such as used in eyeglasses use variations on the wire-grid polariser at the molecular level. In this design, if the electric field is parallel to the conducting path, the photon is reflected back in the direction from which it arrived (with high probability). The remaining light is now polarised so that the photons' electric field is perpendicular to the conductive path in the filter.

If you have more space to generate your polarised light (and it doesn't have to balance on your nose), you can use other methods like prisms or birefringent crystals to separate out different polarisations into different directions. These tend to be non-magnetic materials, so it seems to me that they act on the electric field of the photon to divert it in one direction or the other.
 

Offline RTCPhysics

  • Full Member
  • ***
  • Posts: 66
  • Thanked: 4 times
    • View Profile
Re: Polarisation of visible light.
« Reply #8 on: 25/10/2014 16:13:19 »
Thanks, ChiralSPO. The conclusion that can be drawn from your post, is that the magnetic field vector is as important as the electric field vector for the propagation of light.

Thanks also evan_au. I never figured out why the axis of transmission of a polariser was at right angles to the 'vertical slots'. Now I do.

However, if the magnetic and electric field vectors are of equal magnitude and importance, it must be possible, in concept, to create a polariser from a transparent diamagnetic or paramagnetic material, rather than just the transparent electrically conducting ones that exist. Its likely, though, that the electrically active polarisers are easier and cheaper to manufacture, but there could be a gap in the market for a magnetic one!

A final quote from Wikipedia on the subject.

"Faraday discovered that when a plane polarized ray traverses a transparent diamagnetic medium in the direction of the lines of magnetic force produced by magnets or currents in the neighbourhood, the plane of polarization is caused to rotate."

Thanks to you both again. As regards my original question, my understanding now is that the answer lies in the atomic structure of the polariser.
 

Offline acsinuk

  • Sr. Member
  • ****
  • Posts: 236
  • Thanked: 1 times
    • View Profile
    • electricmagnofluxuniverse.blogspot.com
Re: Polarisation of visible light.
« Reply #9 on: 25/10/2014 17:45:02 »
The polariser may be a physical structure filter or electric or magnetic field polarised as electromagnetic light is comprised of a volume of 3D magnetic energy which is why the flux area component waves are at right angles to the forward force motion.
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4131
  • Thanked: 249 times
    • View Profile
Re: Polarisation of visible light.
« Reply #10 on: 26/10/2014 05:01:54 »
Quote
[near] the lines of magnetic force produced by magnets ... the plane of polarization is caused to rotate.
The same happens for organic chemicals - some rotate the plane of polarisation clockwise, and others anticlockwise (and some don't rotate it at all=racemic).

This is one of the signs that are thought to indicate life, because chemicals produced from inorganic sources tend to be racemic, while proteins produced by living things tend to rotate the polarisation of light in the same direction.

These are called chiral molecules (and is probably the origin of the name chiralSPO).
 

The Naked Scientists Forum

Re: Polarisation of visible light.
« Reply #10 on: 26/10/2014 05:01:54 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums