Dalo

Misty concepts

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Dalo    8

The concept of polarization is an established phenomenon associated with the theory of light as a wave. It is all about vibrations and their direction.
It is also quite incompatible with Huygens' Principle which states that every element of a wave contributes to pushing the wave indefinitely until it meets an obstacle.
Huygens was quite adamant about the fact that side processes did not diminish the energy necessary for the wave to keep going forward. It was, as far as he was concerned, the only reasonable assumption that did justice to the idea that light could travel indefinitely through space.
Later on, those side processes, or "wavelets", acquired more pronounced features under Maxwell and those after him. They become identified as an electric and a magnetic field perpendicular to the direction of the light beam, and also at right angle from each other. It was those fields that made it possible for a wave of light to go on indefinitely.

Those fields were themselves conceived as waves and we immediately wonder about their reach. Looking at a beam of light moving horizontally in front of the observer you can but wonder how far those fields reach. After all, we can see those fields from very far away from aside, so they must somehow be emanating some kind of particles that make them visible to the eye even from very large distances.
A very straightforward explanation is that the beam is made visible through the scattering effect of dust particles in all directions.
That would mean that the forward direction of the beam is at the same time accompanied by side waves that also reflect light in all directions sideways. Something Huygens found undesirable.
It is also very difficult to understand how those fields could at the same time be moving forward and reach to infinity, or at least to large distances, sideways.

Still, this is the view that is accepted by all scientists, and I will leave it to others to show me the errors of my ways. After all, I am not a physicist.

This is also the view used by Sir Bragg in one of his very instructive black and white short films. where he explains polarization and the fact that a beam of light becomes sometimes invisible horizontally, and other times vertically, depending on how we rotate a Polaroid filter in front of the light source.

I will only mention in passing my lack of understanding how vibrations can make objects visible or invisible, which is what Bragg is implying. When light is vibrating vertically we do not see the beam when we are facing it, but only its reflection on the mirror above, and vice versa when it is vibrating vertically.
I wonder if it should be possible to make an object invisible just by the right choice of vibration. Also, I cannot remember any theory of vision that uses vibrations as a means of activating optical cells.

Anyway, yesterday was quite misty, at least in the sky where no individual clouds could be distinguished. It was all wet and gray, and the sun could not be seen anywhere.
That made me think of the way the light beam disappears in Bragg's experiment. Obviously the light is still present, since it was still day. But no sun rays or beams could be seen anywhere, the scattering of light being as perfect as can be.
And I wonder if something like that is not responsible for the appearing and disappearing of the beam in Sir Bragg's experiment. After all, the light is still there overall. Even in places where it appears not to be.


 

Edited by Dalo

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HallsofIvy    52
On ‎11‎/‎11‎/‎2017 at 4:46 PM, Dalo said:

The concept of polarization is an established phenomenon associated with the theory of light as a wave. It is all about vibrations and their direction.


It is also quite incompatible with Huygens' Principle which states that every element of a wave contributes to pushing the wave indefinitely until it meets an obstacle.
Huygens was quite adamant about the fact that side processes did not diminish the energy necessary for the wave to keep going forward. It was, as far as he was concerned, the only reasonable assumption that did justice to the idea that light could travel indefinitely through space.
Later on, those side processes, or "wavelets", acquired more pronounced features under Maxwell and those after him. They become identified as an electric and a magnetic field perpendicular to the direction of the light beam, and also at right angle from each other. It was those fields that made it possible for a wave of light to go on indefinitely.  Those fields were themselves conceived as waves and we immediately wonder about their reach. Looking at a beam of light moving horizontally in front of the observer you can but wonder how far those fields reach. After all, we can see those fields from very far away from aside, so they must somehow be emanating some kind of particles that make them visible to the eye even from very large distances..

  Not quite.  The "wavelets", as you call them, are not "an electric and magnetic field".  They are waves in the ambient electro- magnetic field that pervades space.

Quote

A very straightforward explanation is that the beam is made visible through the scattering effect of dust particles in all directions.


That would mean that the forward direction of the beam is at the same time accompanied by side waves that also reflect light in all directions sideways. Something Huygens found undesirable.
It is also very difficult to understand how those fields could at the same time be moving forward and reach to infinity, or at least to large distances, sideways.

Still, this is the view that is accepted by all scientists, and I will leave it to others to show me the errors of my ways. After all, I am not a physicist.

  No, that is NOT "the view that is accepted by all scientists".  Again,  light is NOT the "electric and magnetic  fields".  The electric and magnetic fields do NOT "move forward" or move to the side.  It is the waves in the electromagnet field (and there is only one such field, not separate "electric" and "magnetic" fields) that move, not the field itself.

 

This is also the view used by

Sir Bragg in one of his very instructive black and white short films. where he explains polarization and the fact that a beam of light becomes sometimes invisible horizontally, and other times vertically, depending on how we rotate a Polaroid filter in front of the light source.

I will only mention in passing my lack of understanding how vibrations can make objects visible or invisible, which is what Bragg is implying. When light is vibrating vertically we do not see the beam when we are facing it, but only its reflection on the mirror above, and vice versa when it is vibrating vertically.
I wonder if it should be possible to make an object invisible just by the right choice of vibration. Also, I cannot remember any theory of vision that uses vibrations as a means of activating optical cells.

Anyway, yesterday was quite misty, at least in the sky where no individual clouds could be distinguished. It was all wet and gray, and the sun could not be seen anywhere.
That made me think of the way the light beam disappears in Bragg's experiment. Obviously the light is still present, since it was still day. But no sun rays or beams could be seen anywhere, the scattering of light being as perfect as can be.
And I wonder if something like that is not responsible for the appearing and disappearing of the beam in Sir Bragg's experiment. After all, the light is still there overall. Even in places where it appears not to be.


 

 

Edited by HallsofIvy

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Dalo    8

@HallsofIvy

I was hoping you would go beyond the textual clarifications and address the issues themselves. Especially the one I formulated in the following lines:

On 11/11/2017 at 11:46 PM, Dalo said:

I cannot remember any theory of vision that uses vibrations as a means of activating optical cells.

I have to admit that I find polarization the most puzzling subject in all of physical optics. It is not so much a matter of understanding, since there is nothing really complicated about the explanations given, but more about how unconvincing they sound to my ears.

What I understand of vision, and without being an expert I can say that I have studied it quite thoroughly, I just cannot seem to rime it with the explanations given. I hoped that this forum would help overcome my skepticism.

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studiot    1190

It would help if we could narrow down the focus on your question.

 

Are you seeking a biochemical explanation of the mechanism of vision?

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Dalo    8
7 minutes ago, studiot said:

It would help if we could narrow down the focus on your question.

 

Are you seeking a biochemical explanation of the mechanism of vision?

No, I need a lecture neither on vision nor on Polarization, but an explanation of how they can be fitted with each other. Do vision theories support the physical explanation, do they contradict them? 

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Strange    2543

What is the connection between polarisation and vision? I didn’t think the photoreceptors in the eye were sensitive to polarisation. (At least, not the human eye.) Am I mistaken?

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Dalo    8
10 minutes ago, Strange said:

What is the connection between polarisation and vision? I didn’t think the photoreceptors in the eye were sensitive to polarisation. (At least, not the human eye.) Am I mistaken?

You are mostly right:

"Most animals do not have eyes that are sensitive to polarized light or behavioral activities that
require polarized light. Wilhelm Karl von Haidinger (born February 5, 1795, died March 19,
1871) published a paper in 1844 announcing his discovery that the human eye does perceive linearly
polarized light. This visual sensation is manifested as two opposing paddle-shaped yellow regions
with blue areas orthogonal to the yellow. This pattern, known as Haidinger’s brushes, is best seen
when looking at a highly polarized white background." 
Dennis H. Goldstein, Polarized Light,  2011.

Still, what is certain, is that we can see its effects. I find the following quote from the same author very interesting:

"light reflecting off an object on the dashboard reflects back to the inner surface of the windshield and then to the driver, and is polarized horizontally. The sunglasses eliminate this image as if by magic." (p.7) [my emphasis]

We have all seen how reflections on a window seem to disappear when we are using Polaroid filters or glasses. Even more mysterious are the examples of polarization where the effects are seen with the naked eye, as the example given above by sir Bragg.

"

Edited by Dalo

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swansont    6251
On 11/11/2017 at 5:46 PM, Dalo said:

The concept of polarization is an established phenomenon associated with the theory of light as a wave. It is all about vibrations and their direction.
It is also quite incompatible with Huygens' Principle which states that every element of a wave contributes to pushing the wave indefinitely until it meets an obstacle.

There's no incompatibility. Polarization tells you the direction of the E and B field in the wave, but that doesn't affect the propagation.

On 11/11/2017 at 5:46 PM, Dalo said:

 Those fields were themselves conceived as waves and we immediately wonder about their reach. Looking at a beam of light moving horizontally in front of the observer you can but wonder how far those fields reach. After all, we can see those fields from very far away from aside, so they must somehow be emanating some kind of particles that make them visible to the eye even from very large distances.
A very straightforward explanation is that the beam is made visible through the scattering effect of dust particles in all directions.
That would mean that the forward direction of the beam is at the same time accompanied by side waves that also reflect light in all directions sideways. Something Huygens found undesirable.

You can only see light from the side of a beam if some of the light is scattering. Again, not incompatible with Huygens.

On 11/11/2017 at 5:46 PM, Dalo said:

This is also the view used by Sir Bragg in one of his very instructive black and white short films. where he explains polarization and the fact that a beam of light becomes sometimes invisible horizontally, and other times vertically, depending on how we rotate a Polaroid filter in front of the light source.

Light doesn't become invisible. If you send light through a polarizer at 90º, that light is absorbed. It no longer exists.

On 11/11/2017 at 5:46 PM, Dalo said:

 Anyway, yesterday was quite misty, at least in the sky where no individual clouds could be distinguished. It was all wet and gray, and the sun could not be seen anywhere.

That made me think of the way the light beam disappears in Bragg's experiment. Obviously the light is still present, since it was still day. But no sun rays or beams could be seen anywhere, the scattering of light being as perfect as can be.

It's called diffusion. (A device where you do this on purpose is a diffuser)

2 minutes ago, Dalo said:

 "light reflecting off an object on the dashboard reflects back to the inner surface of the windshield and then to the driver, and is polarized horizontally. The sunglasses eliminate this image as if by magic." (p.7) [my emphasis]

But, of course, it's not magic. 

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Dalo    8
16 minutes ago, swansont said:

But, of course, it's not magic. 

I couldn't agree more.

Concerning your other remarks. They are also very convincing and I need to review my arguments and maybe views on the matter.

I propose to concentrate on the relation vision and polarization.

28 minutes ago, swansont said:

Light doesn't become invisible. If you send light through a polarizer at 90º, that light is absorbed. It no longer exists.

This would solve at least half of the puzzle: we do not need to see that which does not exist anymore.

If we continue this line of reasoning, then the fact that we can see polarized light has nothing to do with it being polarized, and we do not need to try and explain how vibrations influence vision.

I am not sure that is the right explanation, but this remark seems to imply it. 

Edited by Dalo

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studiot    1190
20 minutes ago, Dalo said:

If we continue this line of reasoning, then the fact that we can see polarized light has nothing to do with it being polarized, and we do not need to try and explain how vibrations influence vision.

Keep going, you are getting there.

 

:)

 

One term I think not yet mentioned is glare.

Polarisation filters work for improving vision because they cut down glare.

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swansont    6251
32 minutes ago, Dalo said:

 I propose to concentrate on the relation vision and polarization.

This would solve at least half of the puzzle: we do not need to see that which does not exist anymore.

If we continue this line of reasoning, then the fact that we can see polarized light has nothing to do with it being polarized, and we do not need to try and explain how vibrations influence vision.

I am not sure that is the right explanation, but this remark seems to imply it. 

And what polarizers do is selectively filter based on polarization. Light that has reflected off of a surface is preferentially polarized parallel to that surface. A polarizer at 90º gets rid of that light but only half of the light that is randomly polarized.

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Dalo    8
10 minutes ago, swansont said:

And what polarizers do is selectively filter based on polarization. Light that has reflected off of a surface is preferentially polarized parallel to that surface. A polarizer at 90º gets rid of that light but only half of the light that is randomly polarized.

how does that relate to the fact that we can see objects/reflections or not?

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swansont    6251
1 minute ago, Dalo said:

how does that relate to the fact that we can see objects/reflections or not?

Could you clarify this? Give an example, perhaps?

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Dalo    8
16 minutes ago, swansont said:

Could you clarify this? Give an example, perhaps?

Sir Bragg

Sometimes we see the beam, sometimes we do not. At least, not at the same location.

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swansont    6251
1 hour ago, Dalo said:

Sir Bragg

Sometimes we see the beam, sometimes we do not. At least, not at the same location.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polar.html

At Brewster's angle you get no reflection of one polarization because all the light is transmitted. All of the reflected light is the other polarization

https://en.wikipedia.org/wiki/Brewster's_angle

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Dalo    8
1 hour ago, swansont said:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polar.html

At Brewster's angle you get no reflection of one polarization because all the light is transmitted. All of the reflected light is the other polarization

https://en.wikipedia.org/wiki/Brewster's_angle

I reread the info just to make sure I had not missed anything, but I am afraid that it still does not explain why, for instance, we see the beam through the water in one position of the filter, and its mirror reflection in another position.

Say the beam is totally transmitted in one case, then how come we do not see its reflection on the mirror? What could possibly stop this reflection?

In the other case, how could there be a mirror reflection without the beam that is being reflected?

Maybe you could show me how the rules can be applied to the example in question?

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Dalo    8
16 hours ago, swansont said:

Light doesn't become invisible. If you send light through a polarizer at 90º, that light is absorbed. It no longer exists.

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.

 

16 hours ago, swansont said:

It's called diffusion. (A device where you do this on purpose is a diffuser)

I must admit that I see very little difference between the diffusion caused by mist and the way the water looks when the beam is polarized. In both cases, it seems like the beam, even if present, is much too "diluted" to leave a clear trace.

 

Like I said, I find light polarization very puzzling, and I hope you will be able to clarify it for me.

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swansont    6251

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

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Dalo    8
8 minutes ago, swansont said:

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

Okay. I find it very strange but okay.

You shine a light through a water tank with water that has been made murky to make light beams  visible. With no polarizing filter and the light turned on you can see the beam crossing the tank and reflected on a mirror above. 

When using a polarizing filter you get to see, depending on how you hold the filter, either the beam through the water, or its reflection on the mirror. Never both at the same time.

May I also remind you that we are talking about a lecture of the eminent physicist sir Bragg?

17 hours ago, studiot said:

Keep going, you are getting there.

 

:)

 

One term I think not yet mentioned is glare.

Polarisation filters work for improving vision because they cut down glare.

I apologize for not reacting sooner. Your remark got lost in the discussion following it.

I just want to say that disconnecting polarization and vision still does not explain Bragg's experiment.

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swansont    6251
1 hour ago, Dalo said:

Okay. I find it very strange but okay.

You shine a light through a water tank with water that has been made murky to make light beams  visible. With no polarizing filter and the light turned on you can see the beam crossing the tank and reflected on a mirror above. 

When using a polarizing filter you get to see, depending on how you hold the filter, either the beam through the water, or its reflection on the mirror. Never both at the same time.

OK, I watched a little more of the video. 

The polarization of the light can effect how it scatters in the tank. Dipoles do not radiate along their axis, so horizontally-polarized light can't be scattered  out the side of the tank — it goes vertically. Vertically polarized light can't scatter up or down.

He explains this at about the 9:00 mark.

 

 

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Dalo    8
Just now, swansont said:

OK, I watched a little more of the video. 

The polarization of the light can effect how it scatters in the tank. Dipoles do not radiate along their axis, so horizontally-polarized light can't be scattered  out the side of the tank — it goes vertically. Vertically polarized light can't scatter up or down.

He explains this at about the 9:00 mark.

With all due respect you are repeating what he said and what can be found in all textbooks. I just do not see how it can be applied to the situation here.

How can a beam that is reflected on a mirror not be visible? Even if it scatters vertically it should still be visible from aside. The same way, if it scatters horizontally it should still be visible from above.

After all, a square or rectangle or cube can be understood as horizontal planes or vertical planes stacked together.

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swansont    6251
1 hour ago, Dalo said:

With all due respect you are repeating what he said and what can be found in all textbooks. I just do not see how it can be applied to the situation here.

How can a beam that is reflected on a mirror not be visible? Even if it scatters vertically it should still be visible from aside. The same way, if it scatters horizontally it should still be visible from above.

After all, a square or rectangle or cube can be understood as horizontal planes or vertical planes stacked together.

Light doesn't scatter in the direction of the mirror. There is no light to reflect.

If it scatters vertically (and is therefore reflected) then it doesn't scatter horizontally. You get one or the other, but not both, when the light is polarized.

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Dalo    8
2 hours ago, swansont said:

Light doesn't scatter in the direction of the mirror. There is no light to reflect.

If it scatters vertically (and is therefore reflected) then it doesn't scatter horizontally. You get one or the other, but not both, when the light is polarized.

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

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Strange    2543
4 hours ago, Dalo said:

Even if it scatters vertically it should still be visible from aside. The same way, if it scatters horizontally it should still be visible from above.

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! :)

 

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