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Old 09-01-2016, 11:40 AM #1
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Default Why do we determine light to be "polarized" by the plane of the electric field?

Title pretty much says it all: why have we determined that the plane of the electric field of an electromagnetic wave should be the plane of polarization, rather than the plane of the magnetic field?

Is it simply because there needs to be a standard way to determine the rotation of light, or does the electric field play a more significant role than the magnetic field? If so, how?

I'm still kinda new to this, and I figured that this was worth asking.


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Old 09-01-2016, 02:59 PM #2
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

Same thing for the polarization of RF or radio waves too, but no idea why.
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Old 09-01-2016, 03:50 PM #3
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

The choice to use the electric field is purely arbitrary and is just the convention the scientific community has decided to use. It could just as well have been the magnetic field. They are both always at 90 degrees to each other so it doesn't make much difference either way.
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Old 09-02-2016, 01:54 AM #4
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

Indeed, it is basically a choice that has no consequence as long as you are consistent.

We ended up with the electrons having negative charge due to a similar choice at that time: Current was given a direction before we even knew how atoms are built, not what electrons even are.

In case of the electron i could be argued that the wrong choice was made as the actual carriers of charge move in opposite direction of electrical current. Considering electrons negative and nuclei positive charges has worked out just fine though.

In case of EM polarity there is a reasonable argument that comes from RF though: Signals emitted from or received by antenna's pointing up are 'vertical' and those between horizontally/sideways oriented antenna's are 'horizontal'.

The way radio waves reflect off the ionosphere preserves this orientation for long range communications as well: if you send a radio signal from the north pole to the equator it would bounce back and forth arriving with the same polarization at the listening station referenced to terrain, but in case of a vertical polarization the antennas would be at 90 degrees to eachother by that point
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Old 09-04-2016, 06:42 AM #5
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

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Originally Posted by H2Oxide View Post
Title pretty much says it all: why have we determined that the plane of the electric field of an electromagnetic wave should be the plane of polarization, rather than the plane of the magnetic field?

Is it simply because there needs to be a standard way to determine the rotation of light, or does the electric field play a more significant role than the magnetic field? If so, how?

I'm still kinda new to this, and I figured that this was worth asking.
In quantum mechanics they measure the electric field of confined particles, not the magnetic field. When experiments are performed, like Young's experiment, they filter by the electric field instead of the magnetic field. Also, the magnetic field is created by the motion of the electrically charged particle not the other way around.

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Old 09-16-2016, 01:34 AM #6
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

Just saw this thread. Like others have said it could have easily been defined the other way around, but when it comes to EM waves, the forces that the electric field impose on particles are typically much larger than the magnetic forces involved, so the magnetic portion can be considered negligible. Because the electric field portion is so much more significant in most situations, the polarization is described in terms of the electric field.


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Also, the magnetic field is created by the motion of the electrically charged particle not the other way around.
For most things that we interact with in our daily live this is true, but the exceptions are pretty cool. For instance the neutron, which has a net charge of zero, still has an intrinsic magnetic moment! That means they behave like tiny little bar magnets when placed in a magnetic field. Protons also have an intrinsic magnetic moment that is not from it moving around. (I know you're a chemist, so you've probably used the magnetic properties of nuclei countless times if you've ever had to run NMR on your samples )
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Old 09-16-2016, 02:24 AM #7
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

There is a big difference between magnetism at quantum level (spin) and at macro level (ferro/dia/para magnetism).

As for things with zero charge but magnetic moments, i'd guess actual magnets (the type that go on your fridge) would be a prime example of something with no electrical charge but very noticable magnetic moment.
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Old 09-16-2016, 06:09 AM #8
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

In general more attention is paid to the electric field than the magnetic field of electromagnetic waves because detectors such as the eye, photographic film, CCDs, RF receivering devices and so on, interact with the electric field.

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Old 09-16-2016, 07:16 AM #9
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

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Originally Posted by styropyro View Post
Just saw this thread. Like others have said it could have easily been defined the other way around, but when it comes to EM waves, the forces that the electric field impose on particles are typically much larger than the magnetic forces involved, so the magnetic portion can be considered negligible. Because the electric field portion is so much more significant in most situations, the polarization is described in terms of the electric field.



For most things that we interact with in our daily live this is true, but the exceptions are pretty cool. For instance the neutron, which has a net charge of zero, still has an intrinsic magnetic moment! That means they behave like tiny little bar magnets when placed in a magnetic field. Protons also have an intrinsic magnetic moment that is not from it moving around. (I know you're a chemist, so you've probably used the magnetic properties of nuclei countless times if you've ever had to run NMR on your samples )
Yes very cool indeed! I have run hundreds of NMRs, both 1H and 13C through those tough 6 semesters of organic chemistry. haha
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Old 09-16-2016, 08:54 AM #10
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

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Humans have the ability to perceive polarized light. We are not aware of this sense and do not use it, but it is there. Actually it has been known for sometime but not generally. Few people, even in the scientific community, are aware that humans can perceive/discern the polarisation of light with the naked eye.

The unaided human eye can tell not only if the light you look at is strongly polarized or not, but also if it is linearly polarized or circularly polarized and, moreover, in which direction it vibrates or rotates.

Is a very cool power of perception that most people do not even know they have plus you can train yourself to use it.

Information on human visions ability to see polarized light and how to practice recognizing polarized light see: https://www.polarization.com/haidinger/haidinger.html

A recent research paper on the subject appears here: Perceiving polarization with the naked eye: characterization of human polarization sensitivity | Proceedings of the Royal Society of London B: Biological Sciences

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Old 09-17-2016, 01:17 AM #11
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

That's actually quite interesting. Polarized light was never much of an issue for people, but since LCDs came about we are all exposed to it all of the time.

Personally i've never noticed anything like what's described but haven't been looking for it either.

Would it make any practical difference if your for example rotate a computer screen 90 degrees? I think the most notable issue with that comes with cleartype rendering of fonts since the sub-pixels are suddenly vertical and your text usually is horizontal giving some issues.

With devices like smartphones and tablets you could use them in either orientation. The pixel density on those things is so high i cannot really see the subpixel rendering issues anymore, but would polarization matter?
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Old 09-17-2016, 03:26 AM #12
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

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That's actually quite interesting. Polarized light was never much of an issue for people, but since LCDs came about we are all exposed to it all of the time.

Personally i've never noticed anything like what's described but haven't been looking for it either.

Would it make any practical difference if your for example rotate a computer screen 90 degrees? I think the most notable issue with that comes with cleartype rendering of fonts since the sub-pixels are suddenly vertical and your text usually is horizontal giving some issues.

With devices like smartphones and tablets you could use them in either orientation. The pixel density on those things is so high i cannot really see the subpixel rendering issues anymore, but would polarization matter?
It takes a little practice, training yourself to see it---it is a subtle thing, more faint than the graphic representations in the article.

You can't concentrate on it, or think about it, because the moment your brain realizes it's not ‘reality', you can't do it anymore. When you do see it, the effect tends to vanish quickly, in a few seconds, as the brain processes away this “useless” information. It requires practice and just the right amount of focus. It requires attentiveness.

When polarized light hits the human eye, a figure called Haidinger's Brush appears. The ‘brush' resembles a pair of crisscrossed bowties; one yellow, one blue. The blue bow runs parallel to the axis of light's polarization, and the yellow is perpendicular. The bows will rotate with the polarization, allowing you to know if it's shifting.

You can see Haidinger’s brushes if you look at a blank white portion of an LCD screen on a computer, tablet or phone. Tilt your head from side to side and faint yellow brushes should become visible. With practice, you can then see them in the blue parts of the sky at 90 degrees from the sun i.e. with your back to the sun looking straight up when the sun is at the horizon--at sun up and sun down.

Here is another small bit about it: https://en.wikipedia.org/wiki/Haidinger%27s_brush

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Old 09-18-2016, 01:28 AM #13
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

I can't see it on any of the LCD screens i have, or perhaps i don't notice it any more since i'm so used to looking into those things.

Perhaps it is more visible when it's changing, like rotating a piece of polarizing foil over a uniform light source - i'm kind of inclined to try something like that just to see if i can see this or not, even if it has no practial application.
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Old 09-18-2016, 04:24 AM #14
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

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I can't see it on any of the LCD screens i have, or perhaps i don't notice it any more since i'm so used to looking into those things.

Perhaps it is more visible when it's changing, like rotating a piece of polarizing foil over a uniform light source - i'm kind of inclined to try something like that just to see if i can see this or not, even if it has no practial application.
Either that or you have seen it so often that has become part of what you consider a "normal white screen" and don't recognize it any longer when looking at a screen.

I spend a lot of time looking at device screens also---not sure if it does or does not impact seeing the Haidinger's Brush polarization patterns. For sure you need to be looking at an all white screen. I can see it but it is a very very subtle faint ghost like thing--you see it and then you don't thing ---a fleeting image canceled out quicky by the brain. If see it or think you see it, as you slowly tilt your head to the left and right and the images remains oriented in the same irection/doen't move---that's it no an artifact created b the brain.
Here is a white page with a graphic of what the "brush" looks like, albeit an exeraggated much brighter representation, in the upper left hand corner --might help jump start your ability to see/recognize it. Clue: Haidinger's brush always appears in the center of the vision.
See:https://longerthoughts.files.wordpre...gers_brush.jpg

Is easier for me to see the "brush" in the sky when it is very polarized. Twilight at dawn and dusk---looking straight up 90 degree's fron the sunlight at the darkest blue part of the sky.

Here is an article that is easy to understand with a lot of literature citations. The article contains excellent simple graphics, written good explainations of the topic, outlines patterns of polarization in nature, and some of practical uses animals make of polarized light because of their ability to see it.
See: Animal Communication: Web Topic 5.2

Consider polarized light vision evolutionary sunglasses. Many animals, such as fish, insects, birds, crabs, and even shrimps -- have wonderfully well-tuned polarization vision to help them solve problems and perform their daily tasks.

There are several human practical applications that have been developed most of them practice of medicine related - example: people with certain kinds of strabismus, or “turning eye,” can be trained to view objects with the correct part of their eye by lining up the Haidinger’s brush with the object they are trying to look at.

Interesting human use of polarization perception --- aid to early navigation. It is thought the Vikings used polarized light to navigate their ships. Scietists measured the polarization pattern of the entire sky under a range of weather conditions during a crossing of the Arctic Ocean on the Swedish icebreaker Oden. The researchers were surprised to find that in foggy or totally overcast conditions the pattern of light polarization was similar to that of clear skies and could have have provided Viking navigators with useful information.
See: Did Vikings navigate by polarized light? : Nature News

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Old 09-19-2016, 01:46 AM #15
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Default Re: Why do we determine light to be "polarized" by the plane of the electric field?

I don't doubt it exists, i just have never noticed it nor can i see anyhing of the sort when looking at an LCD screen displaying only a white background. Or maby my eyes actually do see it but my brain doesnt register it anymore after a long time of spending many hours a day looking into lcd screens with light themes, who knows

I am well aware that many animals can see polarization from insects to vertabrates and use it for all kinds of purposes including navigation.

I doubt being able to see it is a major evolutionairy drive for humans though. It certainly may have applications when navigating the oceans, but humans have not been doing that for long enough to create an evolutionary push for the trait.

The vikings using crystals do it it further support that the unaided eye is not very good at it.

A more generic ability to determine the place of the sun in the sky even in overcast conditions could be useful for navigation even on land if the surroundings lack any landmarks.

Virtually all landscapes on earth have some features that can serve as references so people rarely run around in circles by lack of them. It might happen in flat deserts, on saltplanes and such, but few people venture onto those and survive without modern technology.

I guess it's comparable to being sensitive to magnetic fields: For migratory birds it may be very useful to sense the earths magnetic field so they can migrate in the same direction even at night. Humans have no magnetic sense and need to rely on a compass (or nowadays gps) to figure out their heading without any visual clues.
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