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Dalo

Misty concepts

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21 minutes ago, Dalo said:

It sounds like you understand it no more than I do.

Right. I'm completely lost when I go into my lab and try and apply these concepts.

12 minutes ago, Strange said:

Light itself is not visible, only when it bounces off something’s nag and reaches they eye. If light is scattered vertically it can’t be seen from the side, only from the top. (And conversely for light scattered sideways)

in either case most light will not be scattered and will pass through and hit the mirror.

I’m afraid I haven’t (can’t) watch the video so can’t be sure if that answer helps! :)

 

This is one of the things that threw me at first, since the setup is not clear in the video at the time stamp of the link. The mirror is above the tank, at 45 degrees to the horizontal, so the camera can see light that is scattered vertically, which will then reflect into the camera. We're looking for H or V scattering, perpendicular to the light propagation.

But since this is dipole scattering and dipoles don't radiate along their axis, H or V polarization will not scatter in the direction of the polarization. Thus H or V light will scatter and be visible only in the plane perpendicular to the polarization.

But apparently I don't understand this.

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Just now, swansont said:

Right. I'm completely lost when I go into my lab and try and apply these concepts.

I would certainly hope not. I am not doubting your expertise but simply saying that the theoretical explanation is lacking, at least in the case at hand. I have no reason to doubt calculations made in Optics or Physics in general.

Maybe there is a good explanation, but I have not come across it yet.

 

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1 minute ago, Dalo said:

I would certainly hope not. I am not doubting your expertise but simply saying that the theoretical explanation is lacking, at least in the case at hand. I have no reason to doubt calculations made in Optics or Physics in general.

Maybe there is a good explanation, but I have not come across it yet.

What is lacking in the explanation I gave? Or the one in the video?

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11 minutes ago, swansont said:

Right. I'm completely lost when I go into my lab and try and apply these concepts.

This is one of the things that threw me at first, since the setup is not clear in the video at the time stamp of the link. The mirror is above the tank, at 45 degrees to the horizontal, so the camera can see light that is scattered vertically, which will then reflect into the camera. We're looking for H or V scattering, perpendicular to the light propagation.

But since this is dipole scattering and dipoles don't radiate along their axis, H or V polarization will not scatter in the direction of the polarization. Thus H or V light will scatter and be visible only in the plane perpendicular to the polarization.

But apparently I don't understand this.

OK. That makes perfect sense. 

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27 minutes ago, swansont said:

What is lacking in the explanation I gave? Or the one in the video?

I am afraid that would bring us all the way back to the beginning of this discussion. It has also to do with what @Strangesaid: "Light itself is not visible, only when it bounces off something". For us not to see it from one side would mean that somehow a wall has been erected between the illuminated particles and us. Since we can walk around the water tank, that means that one time the beam is contained between two opaque walls that only open at the top, allowing it to be reflected on the mirror. The other time, there is a wall between the mirror and the water and we see the beam only from aside.

The polarizations effects we are seeing here are caused either by natural minerals (tourmalin, some form of calcite), or by the Polaroid sheets Land had patented. Maybe that an explanation can be found which would explain how come these minerals or sheets have these effect, but I admit my ignorance on this point. What I have read did not make much sense to me and I would really like to understand.

34 minutes ago, swansont said:

The mirror is above the tank, at 45 degrees to the horizontal

This is irrelevant. Instead of the mirror you can simply have an observer looking down, and it won't matter at what angle he holds his head.

Edited by Dalo

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15 minutes ago, Dalo said:

I would certainly hope not. I am not doubting your expertise but simply saying that the theoretical explanation is lacking, at least in the case at hand. I have no reason to doubt calculations made in Optics or Physics in general.

Maybe there is a good explanation, but I have not come across it yet.

 

Non-polarized light is made up of waves for which the electric wave is oscillating all directions.  A polarized filter prevents all of them except the one oscillating in the right plane from passing through.  This is due to the way that the molecules line up in the polarizing filter.   

Something like this:

polarisation1.gif

( of course in this image, only some of the waves in the non-polarized light are shown).

When light is scattered, like they are in the particles of the water, they are only scattered in a direction along the plane perpendicular to the direction of the wave is oscillating. Thus the polarized light in the above image can only be scattered along the left and right plane and not in the up and down one.  If the polarizing filter were to be rotated 90 degrees, it would only let the light which oscillated in the horizontal through, and these waves could only be scattered in the up and down plane and not the left and right one.

When non-polarized light passes through the water, each wave is scattered according to the orientation of that wave's oscillation.  Since the beam is a mixture of light waves of various orientations, some waves will be scattered in the direction of our eyes no matter from which direction we look at the beam passing through the water.

If the light is polarized before entering the water, Then waves that oscillate in only one plane enter the water, and they can only be scattered along one plane.  If the light is polarized vertically, then it will only be scattered horizontally, and you only see this scattered light if you are to the side of the tank.  If it is polarized horizontally, the light is only scattered vertically, and thus you you can only see this light if you are looking at the tank from the top or bottom, ( our have this light reflected to you by a properly placed mirror.)

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Horizontal and vertical are relative terms. We could look at the setup lying down on one side, and the horizontal and vertical wave would switch places. To be sure, we could also take a picture from different angles.

I like the drawing even if it makes me curious. In it, polarizing sheets are not different from gratings. Still, the effects are quite different.
That is one difficulty I have with all drawings I came across of polarizing filters. Except Land's, his drawings were incomprehensible to me. Maybe you could help?

Edited by Dalo

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I think this may be what you are trying to describe.

 

plpolar.gif.be8e0a92cdaae45129424b344c706eb2.gif

 

However I should observe that terminology has been rather loose here.

The polarisation discussed so far is plane polarisation.

There is also circular and elliptic polarisation where the direction is constantly changing.

 

cpolar1.jpeg.a9a922eb58227a36249d13f42e1b6f43.jpeg

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1 hour ago, Dalo said:

 The polarizations effects we are seeing here are caused either by natural minerals (tourmalin, some form of calcite), or by the Polaroid sheets Land had patented. Maybe that an explanation can be found which would explain how come these minerals or sheets have these effect, but I admit my ignorance on this point. What I have read did not make much sense to me and I would really like to understand.

It would have helped greatly if you had asked how polarizer sheets worked, if that's what you wanted to learn. It's really hard to answer questions not asked.

1 hour ago, Dalo said:

This is irrelevant. Instead of the mirror you can simply have an observer looking down, and it won't matter at what angle he holds his head.

It was not for your edification. It was for Strange, who can't view the video (which is why we prefer not to rely on videos in these situations)

It actually will matter what angle he holds his head, if that angle is perpendicular to the beam axis. You will start to see light as you move off-axis. The extinction is for vertical light only, since the intensity will follow a dipole pattern.

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Just now, swansont said:

It actually will matter what angle he holds his head, if that angle is perpendicular to the beam axis. You will start to see light as you move off-axis. The extinction is for vertical light only, since the intensity will follow a dipole pattern.

I must say that is not my experience as I looked down on the beam (that I could not see from aside).

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1 hour ago, Dalo said:

Horizontal and vertical are relative terms. We could look at the setup lying down on one side, and the horizontal and vertical wave would switch places. To be sure, we could also take a picture from different angles.

The description is relative to the apparatus, and following the speaker's coordinate system.

1 hour ago, Dalo said:

I like the drawing even if it makes me curious. In it, polarizing sheets are not different from gratings. Still, the effects are quite different.

That's just the schematics of the drawings. They are not the same things.

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1 hour ago, Dalo said:

Horizontal and vertical are relative terms. We could look at the setup lying down on one side, and the horizontal and vertical wave would switch places. To be sure, we could also take a picture from different angles.

I like the the drawing even if it makes me curious. In it, polarizing sheets are not different from gratings. Still, the effects are quite different.
That is one difficulty I have with all drawings I came across of polarizing filters. Except Land's, his drawings were incomprehensible to me. Maybe you could help?

When I used horizontal and vertical, it was relative to the image as shown on the screen or relative to the video frame in the video.  The analogy to a grating is pretty spot on, in what ways are you claiming that the effects are quite different?

 

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Just now, Dalo said:

I must say that is not my experience as I looked down on the beam (that I could not see from aside).

How can you have an "experience" like that when viewing a video?

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Just now, Janus said:

The analogy to a grating is pretty spot on, in what ways are you claiming that the effects are quite different?

I don't know, ti seems quite obvious that gratings and filters are different things. What makes you think they are the same?

 

1 minute ago, swansont said:

How can you have an "experience" like that when viewing a video?

Like I said before, I have repeated the experiment.

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2 minutes ago, Janus said:

The analogy to a grating is pretty spot on, in what ways are you claiming that the effects are quite different?

I have to disagree. You are not getting diffraction from a polarizer, and they typical diagram (like the one you posted) is actually wrong. The long chains absorb, rather than pass, the light.

http://www.physicsclassroom.com/class/light/Lesson-1/Polarization

"The general rule is that the electromagnetic vibrations that are in a direction parallel to the alignment of the molecules are absorbed."

Just now, Dalo said:

 Like I said before, I have repeated the experiment.

Unsuccessfully, I take it.

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Just now, swansont said:

Unsuccessfully, I take it.

I had exactly the same results as Bragg. 

2 minutes ago, swansont said:

The long chains absorb, rather than pass, the light.

That's the whole question isn't it? Gratings can also be said to pass and absorb the light. The question now is what do polarizers do exactly? And how would you describe them, or draw them?

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2 hours ago, Dalo said:

For us not to see it from one side would mean that somehow a wall has been erected between the illuminated particles and us.

No, it just means it is not bounced off in that direction (because it is polarised, which limits the range of angles at which the light is scattered). 

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Just now, Strange said:

No, it just means it is not bounced off in that direction (because it is polarised, which limits the range of angles at which the light is scattered). 

How big would the elements need to be to get illuminated on only one side? And how ordered would they need to be to all scatter light in the same direction?

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1 hour ago, swansont said:

I have to disagree. You are not getting diffraction from a polarizer, and they typical diagram (like the one you posted) is actually wrong. The long chains absorb, rather than pass, the light.

http://www.physicsclassroom.com/class/light/Lesson-1/Polarization

"The general rule is that the electromagnetic vibrations that are in a direction parallel to the alignment of the molecules are absorbed."

 

I was talking more along the lines of the behavior rather than the mechanism. 

1 hour ago, Dalo said:

How big would the elements need to be to get illuminated on only one side? And how ordered would they need to be to all scatter light in the same direction?

It has nothing to do with the size or ordering of the particles, it has to do with the fact that polarized light will only scatter along a single plane.

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31 minutes ago, Janus said:

It has nothing to do with the size or ordering of the particles, it has to do with the fact that polarized light will only scatter along a single plane.

from itself? That is quite a mystery then? Change of direction without external cause? Or rather indifference to external causes? If light, or photons, or electrons, or waves, or whatever they are, fall on a surface, shouldn't they "bounce off" that surface?

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1 hour ago, Dalo said:

from itself? That is quite a mystery then? Change of direction without external cause? Or rather indifference to external causes? If light, or photons, or electrons, or waves, or whatever they are, fall on a surface, shouldn't they "bounce off" that surface?

That's reflection and not the mechanism for the scattering we are talking about..  The type of scattering that we are dealing with in the experiment from the video isn't the light "bouncing off" the particles. but a different type of interaction. And with this interaction, the polarization of the light effects the direction in which the scattering occurs.

Edited by Janus

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45 minutes ago, Janus said:

That's reflection and not the mechanism for the scattering we are talking about..  The type of scattering that we are dealing with in the experiment from the video isn't the light "bouncing off" the particles. but a different type of interaction. And with this interaction, the polarization of the light effects the direction in which the scattering occurs.

You are making of scattering and polarization an effect caused by the scatterers, or air molecules. But those are the same air molecules that allow unpolarized light to go though them. We only get polarized light with the use of polarizers, be they minerals or technological artifacts like Polaroid filters. 

Edited by Dalo

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52 minutes ago, Dalo said:

You are making of scattering and polarization an effect caused by the scatterers, or air molecules. But those are the same air molecules that allow unpolarized light to go though them. We only get polarized light with the use of polarizers, be they minerals or technological artifacts like Polaroid filters. 

No.  Non-polarized light is a mixture of light waves which oscillate in all possible planes.   When it enters a scattering medium these waves will be subject to being scattered. Some light waves will travel further through the medium before they are scattered.  in the medium is not a strong scatterer, and the total distance traveled through the medium is short enough, much of the light will make it through without being scattered.  If it is a strong scatterer, or the light must travel an extended distance through it, less of the light will make it straight through and more scattered along the way.  

Any light wave scattered will scatter in a direction that is on a plane that is at a right angle to the plane that the electric field is oscillating in. Since non-polarized light contains waves that are oscillating in all possible planes, you will end up with light waves being scattered in all directions. 

When you polarize light by passing it through a filter, you are only letting light waves through that oscillate in a limited direction.  When this light enters a scattering medium, just like above, it is subject to being scattered. And like above,  some waves will travel further before being scattered.  But since the direction in which the light waves are oscillating is restricted, so is the directions in which the light waves will be scattered.   This is what the video is demonstrating.

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9 hours ago, Dalo said:

You are making of scattering and polarization an effect caused by the scatterers, or air molecules. But those are the same air molecules that allow unpolarized light to go though them. We only get polarized light with the use of polarizers, be they minerals or technological artifacts like Polaroid filters. 

No that's not correct. I linked to some explanations earlier — reflection causes polarization. That's used to get polarized light as well. Scattering causes polarization — you can see the effect if you look at the sky at 90º to the position of the sun.

 

14 hours ago, Dalo said:

That's the whole question isn't it? Gratings can also be said to pass and absorb the light. The question now is what do polarizers do exactly? And how would you describe them, or draw them?

"Gratings can also be said to pass and absorb the light" is a pretty lousy description of what gratings do. Gratings cause diffraction which gives rise to interference, and whatever light is absorbed is not preferentially absorbed depending on its polarization, as in a polarizer.

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8 hours ago, Janus said:

No.  Non-polarized light is a mixture of light waves which oscillate in all possible planes.   When it enters a scattering medium these waves will be subject to being scattered. Some light waves will travel further through the medium before they are scattered.  in the medium is not a strong scatterer, and the total distance traveled through the medium is short enough, much of the light will make it through without being scattered.  If it is a strong scatterer, or the light must travel an extended distance through it, less of the light will make it straight through and more scattered along the way.  

Any light wave scattered will scatter in a direction that is on a plane that is at a right angle to the plane that the electric field is oscillating in. Since non-polarized light contains waves that are oscillating in all possible planes, you will end up with light waves being scattered in all directions. 

When you polarize light by passing it through a filter, you are only letting light waves through that oscillate in a limited direction.  When this light enters a scattering medium, just like above, it is subject to being scattered. And like above,  some waves will travel further before being scattered.  But since the direction in which the light waves are oscillating is restricted, so is the directions in which the light waves will be scattered.   This is what the video is demonstrating.

Another masterclass of clarity of thought and reasoning, Janus. +1

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