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Dalo

Misty concepts

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4 hours ago, swansont said:

"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.

I have reread the information on Land's polarizers, and also his patent . This is what I have understood once all kind of technical and chemical details are left out:

1) Very tiny crystals are arranged in linear arrays and separated by very small gaps (smaller than their width). 

2) These crystal arrays absorb electric current (and light).

3) The gaps between the arrays let light pass.

4) The em waves which pass through the filter have (almost) no "wiggle room" in the direction perpendicular to the array.

5) The light that passes through can only be scattered in the direction parallel to the arrays. (Almost) no light is scattered perpendicular to the arrays.

That leads to the following remarks:

a) The nature of the crystals is irrelevant, only their function of absorbing light is. Any opaque element could be used.

b) The light that gets through has to be scattered by some kind of matter to become visible.

c) The direction in which it is scattered is determined not by the position of the scattering elements but by the direction the transmitted em waves have been polarized.

The last point is what makes the whole a mystery to me. It implies that the scatterers might as well be totally absent for the influence they have on the direction the light is scattered.

edit: I have one or two more remarks on the subject, but I have to think (and probably read) more before I place them.

Edited by Dalo

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31 minutes ago, Dalo said:

That leads to the following remarks:

a) The nature of the crystals is irrelevant, only their function of absorbing light is. Any opaque element could be used.

I don't see how that is supported by anything you've presented. Your link explains how the absorption is polarization-dependent, which is critical. Not all materials will have this feature.

Quote

b) The light that gets through has to be scattered by some kind of matter to become visible.

I don't see how that is supported by anything you've presented. The "visibility" of light depends on whether your eye in in the path of the light.

Quote

c) The direction in which it is scattered is determined not by the position of the scattering elements but by the direction the transmitted em waves have been polarized.

The last point is what makes the whole a mystery to me. It implies that the scatterers might as well be totally absent for the influence they have on the direction the light is scattered.

The scattering is not isotropic, and is polarization dependent. This has been explained to you several times. It's one thing to not understand, but quite another to just ignore information.

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16 minutes ago, swansont said:

I don't see how that is supported by anything you've presented. Your link explains how the absorption is polarization-dependent, which is critical. Not all materials will have this feature.

That is theoretically very relevant, and practically insignificant. Light that has been absorbed is out of the game. All we have left is the light that has passed through the gaps.

18 minutes ago, swansont said:

I don't see how that is supported by anything you've presented. The "visibility" of light depends on whether your eye in in the path of the light.

Then "horizontal" is quite a flexible concept. When you are standing in front the water tank and the beam is visible in the water (and not on the mirror), you can still see the beam whether you bend your knees completely or whether you stand on your toes.

21 minutes ago, swansont said:

The scattering is not isotropic, and is polarization dependent. This has been explained to you several times. It's one thing to not understand, but quite another to just ignore information

That is exactly how I understood it: scattering is polarization dependent. If light is polarized vertically, it will be scattered vertically, and vice versa. My conclusion that the air molecules have no influence on the direction of scattering therefore stands.

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6 minutes ago, Dalo said:

That is theoretically very relevant, and practically insignificant. Light that has been absorbed is out of the game. All we have left is the light that has passed through the gaps.

If the light is not being preferentially absorbed, depending on the polarization, then the light passing through will not be polarized.

6 minutes ago, Dalo said:

Then "horizontal" is quite a flexible concept. When you are standing in front the water tank and the beam is visible in the water (and not on the mirror), you can still see the beam whether you bend your knees completely or whether you stand on your toes.

Yes. I already explained this.

6 minutes ago, Dalo said:

That is exactly how I understood it: scattering is polarization dependent. If light is polarized vertically, it will be scattered vertically, and vice versa.

The opposite. If light is polarized vertically, it will not scatter vertically.

6 minutes ago, Dalo said:

My conclusion that the air molecules have no influence on the direction of scattering therefore stands.

You're the only one talking about air molecules. They're irrelevant.

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2 minutes ago, swansont said:

You're the only one talking about air molecules.

I realize that. That is what I find so puzzling.

edit: in the water tank we must of course speak of water (and other) molecules.

Edited by Dalo

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52 minutes ago, Dalo said:

I realize that. That is what I find so puzzling.

edit: in the water tank we must of course speak of water (and other) molecules.

Neither air nor water are the material scattering the light at ~90 degrees to the beam. So why must we speak of either one?

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Just now, swansont said:

Neither air nor water are the material scattering the light at 90 degrees to the beam. So why must we speak of either one?

I suppose you are right, and I must formulate my idea more carefully. We are talking about the particles present in air or water.

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3 hours ago, swansont said:

The opposite. If light is polarized vertically, it will not scatter vertically.

You are right of course. And that is at the same time one of the questions I still have.

Polarization is an effect created by the filter. When em waves collide with an array the light is absorbed. But then, there are also gaps, in fact, there is a gap between each two arrays. How come light does not simply get through?

Edited by Dalo

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On 11/16/2017 at 9:25 PM, 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.

In my own experiment I could see the beam in the water (and not reflected on the mirror) however I stood around the water tank. Not only from the long sides, but also from above and from any side, and however I bent my head.

Since there was no mirror reflection, I shouldn't have been able to see it, should I?

7 hours ago, swansont said:

If the light is not being preferentially absorbed, depending on the polarization, then the light passing through will not be polarized.

I think that is probably the most important point of all the discussion: what does "being preferentially absorbed" mean? When you consider a Polaroid sheet with vertical chains or arrays, do all the vertical light rays that reach the sheet get absorbed, or only those which find the arrays on their way?

Let us not forget that the gaps are generally smaller than the width of the crystals, according to Land. So, what happens to those vertical rays that face gaps directly. Should they also be stopped, like the other vertical rays? If so, how and why?

And if they are not, then only, approximately, half of the vertical rays will be absorbed, which should still make them visible.

Edited by Dalo

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On 16/11/2017 at 10:01 AM, Dalo said:

I have repeated Bragg's experiment as well as I could. I used a led electric torch much less powerful than the lamp he used, but the contrast was high enough to clearly distinguish between different times when the beam was "invisible" and when visible.

I could watch the water tank from all sides except the bottom. In all cases, however the polarizing sheet was set, I could see the beam reflected on the opposite side of the water tank, and on the wall beyond it. The light was therefore not absorbed, certainly not in its totality.

So that  still leaves the question how the beam could go through the water, not be visible to the eye, and still have a reflection on the mirror, or be seen when looked at from above. This while the beam was each time either visible or invisible from both sides.

For the benefit of those of us unsure exactly how your experiment was conducted can you provide sufficient detail to fully appreciate it ?

Edited by studiot

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Besides, I had exactly the same results as in the link. What is the point of your question?

 

for those who can't watch the video:  youtube.com/watch?v=eWbOXAEZyQA&t=615s

Edited by Dalo

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3 hours ago, studiot said:

For the benefit of those of us unsure exactly how your experiment was conducted can you provide sufficient detail to fully appreciate it ?

I apologize for my tone. 

I have not changed anything to Bragg's experiment, and the description I gave is I think accurate.

 

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Just share the Methods section of your paper, or look at a paper with a Methods section to understand what people are asking. 

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18 minutes ago, iNow said:

Just share the Methods section of your paper, or look at a paper with a Methods section to understand what people are asking. 

this is overkill. I will be happy to answer any reasonable question instead of repeating what I have said. My experiment is the same as Bragg's, the only difference is in the light source used.

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On 11/17/2017 at 11:36 PM, Dalo said:

In my own experiment I could see the beam in the water (and not reflected on the mirror) however I stood around the water tank. Not only from the long sides, but also from above and from any side, and however I bent my head.

Since there was no mirror reflection, I shouldn't have been able to see it, should I?

I think that is probably the most important point of all the discussion: what does "being preferentially absorbed" mean? When you consider a Polaroid sheet with vertical chains or arrays, do all the vertical light rays that reach the sheet get absorbed, or only those which find the arrays on their way?

Let us not forget that the gaps are generally smaller than the width of the crystals, according to Land. So, what happens to those vertical rays that face gaps directly. Should they also be stopped, like the other vertical rays? If so, how and why?

And if they are not, then only, approximately, half of the vertical rays will be absorbed, which should still make them visible.

Nobody?

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11 hours ago, Dalo said:

Besides, I had exactly the same results as in the link

So I assumed everything was clear. 

But ...

On 17/11/2017 at 10:36 PM, Dalo said:

In my own experiment I could see the beam in the water (and not reflected on the mirror) however I stood around the water tank. Not only from the long sides, but also from above and from any side, and however I bent my head.

Since there was no mirror reflection, I shouldn't have been able to see it, should I?

Does this mean that you could always see the (scattered) light from all sides and the top, but you couldn't see it in the mirror even though the mirror is angled so it is the same as you looking down?

In other words, when you look down past the mirror you can see the light, but when you look in the mirror you can't? That doesn't make much sense. Maybe the mirror just isn't aligned to see the path of the light? Perhaps you could provide a drawing & description of your setup?

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5 minutes ago, Dalo said:

Nobody?

You do know, right, that it’s Saturday and people do other things than wait with baited breath just to respond to your posts on this board?

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1 hour ago, Strange said:

In other words, when you look down past the mirror you can see the light, but when you look in the mirror you can't? That doesn't make much sense. Maybe the mirror just isn't aligned to see the path of the light? Perhaps you could provide a drawing & description of your setup?

I think you are barking at the wrong tree. I have no problem admitting that my "experiment'" in no way respects the canons of scientificity. It was not made under controlled conditions and no measuring instruments were used. I t is more akin to casual observations than anything else. 

The results I obtained in this non-scientific way resemble very much those obtained by Bragg in his short film. I was able to walk around the water tank, and no, I could not see the beam from above when it was not reflected by the (hand held) mirror.

At least not from the same position as shown By Bragg. In this sense, there was no difference between what he showed and I saw.

There was a difference though, and that is I think the reason why I was not entirely clear: even when the beam was not reflected on the mirror, and even when I could not see it from above from the same position, I could see it from the end of the path of the beam. That is from the position opposite to where the lamp was. My impression was that the beam did not go as far as when it was visible from above, but since I did not measure it, I will not claim that is the case.

I will therefore stick to Bragg's observation that the beam either was reflected on the mirror without being visible from the side, or vice versa. 

That is also all I have assumed in all this discussion. So, if you want to be pedantic and accuse me of presenting a non-scientific experiment, then I will gladly plead guilty. Now stop beating about the bush and say what you have to say.

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On 11/16/2017 at 4:51 AM, swansont said:

It's not clear to me what the setup is in the video

 

15 hours ago, studiot said:

For the benefit of those of us unsure exactly how your experiment was conducted can you provide sufficient detail to fully appreciate it ?

 

8 hours ago, iNow said:

Just share the Methods section of your paper, or look at a paper with a Methods section to understand what people are asking. 

 

2 hours ago, Strange said:

Perhaps you could provide a drawing & description of your setup?

 

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nope. I have nothing more to say.

 

edit: gee! How strange! I wonder who voted me down?

Edited by Dalo

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6 hours ago, Dalo said:

I think you are barking at the wrong tree. I have no problem admitting that my "experiment'" in no way respects the canons of scientificity. It was not made under controlled conditions and no measuring instruments were used. I t is more akin to casual observations than anything else. 

That's fine. I am just trying to clarify exactly what the setup and results are. Some of your comments about what you see are slightly ambiguous so it is hard to comment without being absolutely clear about what results you are getting.

But I realise drawing a diagram and posting it is a lot of work so ...

Quote

I was able to walk around the water tank, and no, I could not see the beam from above when it was not reflected by the (hand held) mirror.

When you say you couldn't see it from above except when reflected by the mirror, is that because you couldn't get to a position where you were looking down?

The mirror shouldn't make any difference. It is, I assume, only there so the light scattered sideways and the light scattered vertically can both be seen from the same place. (Is that correct?)

So I am slightly puzzled by the fact you can see the light scattered vertically in the mirror but not when viewed directly.

Quote

There was a difference though, and that is I think the reason why I was not entirely clear: even when the beam was not reflected on the mirror, and even when I could not see it from above from the same position, I could see it from the end of the path of the beam.

That is from the position opposite to where the lamp was. My impression was that the beam did not go as far as when it was visible from above, but since I did not measure it, I will not claim that is the case.

I would be very surprised if most of the beam did not pass through the water like this. Unless the water is very murky, only a small proportion will be scattered. This might be changed slightly for polarised vs unpolarised light, but should be the same for vertically or horizontally polarised light.

Quote

I will therefore stick to Bragg's observation that the beam either was reflected on the mirror without being visible from the side, or vice versa. 

That is very ambiguous. It is not immediately clear what it means and I can imagine at least two interpretations - and that might change depending on where the mirror is.

Quote

So, if you want to be pedantic and accuse me of presenting a non-scientific experiment, then I will gladly plead guilty. Now stop beating about the bush and say what you have to say.

I am not making any accusations. Just trying to clarify what you see and what you are still questioning (if anything).

If you get the same result as Bragg and you are happy with the explanation of why polarised light is scattered differently depending on the angle of polarisation, then we are done.

If you are still puzzled by the results, then we can explore that as long as you can be clear about what your setup is and exactly what results you get.

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22 hours ago, Dalo said:

I apologize for my tone. 

I have not changed anything to Bragg's experiment, and the description I gave is I think accurate.

 

Let me thank you for answering my question simply and well, +1

There is nothing to apologise for, but as iNow commented, yesterday was Saturday and I had many other calls on my time.

I needed 20 minutes to watch the full video.

I have not seen Bragg in action so thank you for introducing me to this source, the demonstrations, though old, were fascinating and well chosen.
In my eyes he was both clear and understandable in his explanations.
These seemed to me to answer both your questions that have given rise to long threads.

So well done for attempting to replicate the experiments.

Let's try to all stop squabbling and to pin-point your difficulty instead so that it can be sorted.

I cannot believe that you are not interested since you have put so much effort into this.

However remember that it may take a little time for responses so be patient.
I still owe another member the next long installment of longish series about another subject, from last Sunday..

 

:)

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On 11/17/2017 at 5:36 PM, Dalo said:

In my own experiment I could see the beam in the water (and not reflected on the mirror) however I stood around the water tank. Not only from the long sides, but also from above and from any side, and however I bent my head.

Since there was no mirror reflection, I shouldn't have been able to see it, should I?

The only light that's missing is on the axis. The intensity varies as sin^2(angle) with respect to the polarization. Strongest at 90º, dropping to zero at 0º (i.e. they don't radiate along the axis)

On 11/17/2017 at 5:36 PM, Dalo said:

I think that is probably the most important point of all the discussion: what does "being preferentially absorbed" mean? When you consider a Polaroid sheet with vertical chains or arrays, do all the vertical light rays that reach the sheet get absorbed, or only those which find the arrays on their way?

Light that's polarized along the direction of the chains is absorbed in preference to light at 90º

There is no light that doesn't "find the arrays" unless you aren't hitting the sheet.

On 11/17/2017 at 5:36 PM, Dalo said:

Let us not forget that the gaps are generally smaller than the width of the crystals, according to Land. So, what happens to those vertical rays that face gaps directly. Should they also be stopped, like the other vertical rays? If so, how and why?

There is no way to go between the gaps, unless you have a defective or damaged polarizer.

 

On 11/17/2017 at 2:18 PM, Dalo said:

You are right of course. And that is at the same time one of the questions I still have.

Polarization is an effect created by the filter. When em waves collide with an array the light is absorbed. But then, there are also gaps, in fact, there is a gap between each two arrays. How come light does not simply get through?

Light does make it through the gaps. Obviously. But light can't make it through the gaps without interacting with the polymer chains, so the transmission is polarized.

10 hours ago, Dalo said:

I think you are barking at the wrong tree. I have no problem admitting that my "experiment'" in no way respects the canons of scientificity. It was not made under controlled conditions and no measuring instruments were used. I t is more akin to casual observations than anything else. 

The results I obtained in this non-scientific way resemble very much those obtained by Bragg in his short film. I was able to walk around the water tank, and no, I could not see the beam from above when it was not reflected by the (hand held) mirror.

At least not from the same position as shown By Bragg. In this sense, there was no difference between what he showed and I saw.

Your previous posts implied that the results were different.

That's probably a significant source of confusion and frustration.

10 hours ago, Dalo said:

There was a difference though, and that is I think the reason why I was not entirely clear: even when the beam was not reflected on the mirror, and even when I could not see it from above from the same position, I could see it from the end of the path of the beam. That is from the position opposite to where the lamp was. My impression was that the beam did not go as far as when it was visible from above, but since I did not measure it, I will not claim that is the case.

Whether you can see the beam along the axis and from the side depends on how much material is available to scatter the light and how bright it is. Your eyes have a dynamic response. If you are seeing bright light anywhere, you will have difficulty seeing dim light. But if you are only viewing the dim light, you will be able to see it.

 

3 hours ago, Strange said:

 The mirror shouldn't make any difference. It is, I assume, only there so the light scattered sideways and the light scattered vertically can both be seen from the same place. (Is that correct?)

One (small) difference is in viewing angle. With the mirror positioned as in the video, you are looking at the beam with both eyes aligned with the axis of the light propagation. But if you look in person, you can be at a different angle. e.g. if you look down at the beam and you are facing the source, your eyes are each a small angle from vertical, where the light will not be completely extinguished.

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On 11/17/2017 at 3:02 PM, Dalo said:

I have reread the information on Land's polarizers, and also his patent . This is what I have understood once all kind of technical and chemical details are left out:

1) Very tiny crystals are arranged in linear arrays and separated by very small gaps (smaller than their width). 

2) These crystal arrays absorb electric current (and light).

3) The gaps between the arrays let light pass.

4) The em waves which pass through the filter have (almost) no "wiggle room" in the direction perpendicular to the array.

5) The light that passes through can only be scattered in the direction parallel to the arrays. (Almost) no light is scattered perpendicular to the arrays.

That leads to the following remarks:

It's a common mistake to suppose that the polarisers work like a sieve., i.e. the light gets through the gaps between the "wires".

In fact, it's the other way round. The long thin lines of (in Land's example iodine molecules) "short circuit" the electric component of the wave and thus absorb it.

The light that gets through is the like light that is polarised at right angles to the axis along which the molecules are aligned.

 

You can make this even clearer if you use microwaves because, for them, the wires that make the polarise can be made big enough to see.

The fact that you were not aware of that shows that you don't really know what you are talking about.

On that basis, I suggest you do a lot more reading, and rater less writing (In particular less writing that doesn't end in a question mark)

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1 hour ago, studiot said:

Let me thank you for answering my question simply and well, +1

There is nothing to apologise for, but as iNow commented, yesterday was Saturday and I had many other calls on my time.

I needed 20 minutes to watch the full video.

I have not seen Bragg in action so thank you for introducing me to this source, the demonstrations, though old, were fascinating and well chosen.
In my eyes he was both clear and understandable in his explanations.
These seemed to me to answer both your questions that have given rise to long threads.

So well done for attempting to replicate the experiments.

Let's try to all stop squabbling and to pin-point your difficulty instead so that it can be sorted.

I cannot believe that you are not interested since you have put so much effort into this.

However remember that it may take a little time for responses so be patient.
I still owe another member the next long installment of longish series about another subject, from last Sunday..

 

:)

Thank you, I really appreciate the gesture. I find the intensity of the discussion sometimes quite puzzling. The emotions run very high on scientific subjects which should be approached calmly and coldly. After all, even if I am a complete crackpot, it is not like I could put Science in danger by my views. 

1 hour ago, swansont said:

There is no way to go between the gaps, unless you have a defective or damaged polarizer.

 

1 hour ago, swansont said:

Light does make it through the gaps. Obviously. But light can't make it through the gaps without interacting with the polymer chains, so the transmission is polarized.

The second quote makes more sense, even though I would very much like a non-technical explanation of it (I am sure the mathematical arguments will be in order).

 

24 minutes ago, John Cuthber said:

It's a common mistake to suppose that the polarisers work like a sieve., i.e. the light gets through the gaps between the "wires".

I admit that I sometimes made this same mistake. It is tempting to think of the chains as mechanical obstacles to vertical (or horizontal) rays. I suppose it is what comes first to mind in such situations.

The reality seems to be much more complex: when in a vertical (or horizontal) position the chains absorb all vertical (or horizontal) rays., even those which could pass through the gaps. I will ask of you the same question I asked @swansont.

Edited by Dalo

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