Trying to Understand the Polarization of light

  • #1
leodavinci
1
0
I want to understand the concept of polarization of light. I would like someone to clarify how I am thinking about it:

If light is a transverse wave and it is basically an electric field(E) and magnetic field(B) each oscillating in different directions (axes) which are perpendicular to each other say x-axis for E and z-axis for B and the light propagates in the y-axis. Polarization means that that light is filtered by blocking all light except the one traveling in a certain direction.

1) Say for example, that only light that is traveling vertically is allowed, this means that all light traveling horizontally is blocked. Is this right?
2) If light traveling vertically is allowed only, what does this mean for the Electric Field and Magnetic Field. Is one of them blocked? I know that it doesn't make sense to me, because light is made out of both of E and B. Is it the case that when talk about polarization of light then we are talking about the wave itself which means the E and B fields are inside the wave and their directions of oscillation are actual vertical or horizontal directions?
 

Answers and Replies

  • #2
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2022 Award
23,784
15,399
Summary
Polarization of light clarification of concept

I want to understand the concept of polarization of light. I would like someone to clarify how I am thinking about it:

If light is a transverse wave and it is basically an electric field(E) and magnetic field(B) each oscillating in different directions (axes) which are perpendicular to each other say x-axis for E and z-axis for B and the light propagates in the y-axis. Polarization means that that light is filtered by blocking all light except the one traveling in a certain direction.

1) Say for example, that only light that is traveling vertically is allowed, this means that all light traveling horizontally is blocked. Is this right?
2) If light traveling vertically is allowed only, what does this mean for the Electric Field and Magnetic Field. Is one of them blocked? I know that it doesn't make sense to me, because light is made out of both of E and B. Is it the case that when talk about polarization of light then we are talking about the wave itself which means the E and B fields are inside the wave and their directions of oscillation are actual vertical or horizontal directions?

Polarisation refers to the orientation of the E & B fields. It's not to do with the direction of propagation of the light.

To take an example. If light is traveling in the z-direction, say, then unpolarised light may have E fields in any orientation and corresponding B fields in any orientation (not in the z-direction, of course, for waves in vacuum). If you restrict the light so that the E field must be in the x-direction and hence the B field in the y-direction, then that light is polarised (by convention it is said to be polarised in the x-direction).

That's the case of linear polarisation.
 
  • #3
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Insights Author
35,994
4,715
Polarization of light is defined by the orientation of the E-field vector. So if you have it vertically polarized, it means that the E-field is in the vertical direction.

Zz.
 
  • Informative
  • Like
Likes davenn and Dale
  • #6
hexanol
5
1
This has finally illustrated to me the difference between linearly and circularly polarized light. The phrase: "can be thought of as", is surely worthy of discussion some other place. I still need to stew over the 1/4 wave plate and circular polarization business.

Light was not shed on an aspect of this that has always been a mystery to me visually. In the 3 polarizer example, if the material only allows up-down waves to pass at the first filter, how can any there be any left-right to waves reach the third filter if none were allowed to pass the first filter ("can be thought of as" no longer seems to fly)? What is actually happening with the second filter? Or what didn't happen at the first.

The best I have seen of this is lots of hand waving, some smoke & mirrors, a bit of math, and the knowing mention that the wave was "rotated".
 
  • #7
Merlin3189
Homework Helper
Gold Member
1,717
806
Glad to see this video helped a bit. I think it is brilliant and really helped me a lot.

What I think may be hanging you up now, is that a vector can always be replaced by a pair of orthogonal vectors, which totally represent it and can in turn be replaced by a single vector (their sum), which, if nothing has been done to them meanwhile, will be an exact recreation of the original vector.

[That of course (IMO) is mathematical sophistry! The two component vectors never existed - any more than the original vector did! Vectors are just a mathematical way of describing light: and both descriptions, as a single vector and as a pair of vectors, are equally valid.
That (personal view) aside, let's look at the light and how it behaves. ]

12:50 on, shows how light can have its polarisation changed by a linear polariser.
A linear polariser passes all light parallel to its polarisation, and blocks all light perpendicular to its polarisation
What about light at other angles? It treats it as if it were two waves, one parallel and one perpendicular to its polarisation. Then it blocks the one and passes the other, giving you linearly polarised light with a different polarisation from the incoming wave. Just as if it were doing the maths and resolving the incoming vector into two orthogonal vectors.

I think your question is, not the maths, but why the polariser behaves like this.
If you look at the light coming out of each polariser, it is always plane polarised in the preferred direction. (This is shown in the video by the stripes, which run in the blocking direction.) Now I don't know exactly how this works (you'll have to find a quantum physicist for that), but I'll give you my classical semi-understanding.
Light interacts with matter, causing it to slow down, be reflected, refracted and polarised, because it excites electrons. This excitation may depend on the relative orientation of molecules and the electric field of the light. OTOH the re-emitted light depends only on the orientation of the molecules. In a polarising material, the significant molecules are all or mainly aligned in one direction. That, of course, is the arm-waving you mention! And some people describe it as absorption in one direction. I can't see how you can get away without both absorption and emission: both can be achieved by exciting electrons, but I personally can't get the numbers to work when both happen.
You may be interested to watch Sixty Symbols - Polarisation For me this is still arm-waving, but he does say that he could explain it mathematically, if only we could understand him!

Even though I can't give a decent account of it, the alignment of molecules or chemical (ie. electrical) bonds seems pretty likely the key. The main materials for polarisers are polymers with oriented molecules, crystals with very ordered electronic lattices, or grids of conductors (as shown in his experiment with microwaves).

There is, incidentally, another way (at least) of polarising light. That is to use reflection at the interface of two transparent media. You can take microscope slides and tilt them at the Brewster angle for that glass and pass the beam of light through. There is no absorption, but the transmitted and reflected light has opposite polarisation. Doesn't really help that much, as I can't find a satisfying classical explanation and the quantum one seems just as complex as for filters.

I don't know if any of that helps. If I ever locate a decent explanation of how polaristion occurs in any material, I'll post again. Perhaps a Quantum Physicist might attempt an explanation that we could understand - or maybe you'll become one yourself soon! Too late for me: I'm forgetting even that which I used to know (or mistakenly believed I knew, if Unzicker is right!)
 

Suggested for: Trying to Understand the Polarization of light

  • Last Post
Replies
6
Views
208
Replies
7
Views
515
Replies
7
Views
772
Replies
2
Views
535
Replies
2
Views
445
  • Last Post
Replies
4
Views
457
Replies
4
Views
630
Replies
5
Views
622
Replies
9
Views
609
Top