Misty concepts

[studiot]

1 hour ago, Dalo said:

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. 

 

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).

 

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.

It wasn't a gesture, it was sincere.

Bragg explains how the grating works in his video, when he says that there is a regular pattern in crystal so the crystal is anisotropic to the passage of light.

[Dalo]

1 minute ago, studiot said:

Bragg explains how the grating works in his video, when he says that there is a regular pattern in crystal so the crystal is anisotropic to the passage of light.

Could you maybe pinpoint the time at which he does that? It is a long video indeed

[Strange]

1 hour ago, Dalo said:

I will ask of you the same question I asked @swansont.

You ask for "a non-technical explanation of it" even though you are responding to a post where he gave you an excellent non-technical explanation. 

There's no helping some people.

[Dalo]

1 minute ago, Strange said:

You ask for "a non-technical explanation of it" even though you are responding to a post where he gave you an excellent non-technical explanation. 

There's no helping some people.

your childish tantrums are really tiresome.

[Strange]

But if you were to say which parts of John's explanation were not clear to you, it would perhaps be more helpful than simply asking for the explanation you have just been given.

[Dalo]

I have been given a puzzle: the arrays or chains absorb all rays that have the same orientation, even those that we would expect could go right through the gaps. I do not understand how that is possible.

[John Cuthber]

Light often goes through stuff, air, windows, whatever.

The alternatives are that it gets absorbed or that it gets reflected. Polaroid filters don't reflect much so we can focus on absorption.
 

The nature of light- especially polarised light is that it is composed of a magnetic component and an electrical one at right angles to eachother and also to the direction of propagation.

If you put a metal wire in an electric field the electrons in it will respond to the field and move along the wire. The field due to the displaced electrons tends to oppose the original field. If you do that with a field that changes with time the field drags the electrons up and down the wire. But that movement is a current. And a current flowing in a wire will encounter resistance and that transfers energy from the field and converts it to heat.

So a set of conductive wires like a polariser will tend to tend to convert the light energy into heat; that's absorption.

If the wires are mounted at right angles to the field the electrons are not moved so far and so there's a lot less absorption.

[studiot]

Just to reinforce what John Cuthber said.

 

It is only the electric component vector of light that plays a part in these effects.

Bragg also explains this in his video.

 

The simplest way to play with polarisation is Malus' Experiment.

 

https://www.google.co.uk/search?source=hp&ei=ifsRWrRw7IyABt3hqKgO&q=malus+experiment&oq=malus+exper&gs_l=psy-ab.1.0.0j0i22i30k1l5j0i22i10i30k1l2.1809.4225.0.7143.11.11.0.0.0.0.260.1705.1j8j2.11.0....0...1.1.64.psy-ab..0.11.1692...46j0i131k1j0i46k1.0.rJABLl3UJX0

 

Here also is a pic which shows the effect of finer and finer gratings on diffraction.

 

[swansont]

8 hours ago, Dalo said:

 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).

Light can't go through arbitrarily small spaces without interacting. It's why the door on a microwave oven doesn't have to be solid, and yet the microwaves don't leak out.

Quote

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.

Think of it this way. If you tried to walk through a normal doorway carrying a 6 foot pole, could you get through if you held it horizontally? I mean, there's nice big gap there. 

[Dalo]

5 hours ago, John Cuthber said:

Light often goes through stuff, air, windows, whatever.

The alternatives are that it gets absorbed or that it gets reflected. Polaroid filters don't reflect much so we can focus on absorption.
 

The nature of light- especially polarised light is that it is composed of a magnetic component and an electrical one at right angles to eachother and also to the direction of propagation.

If you put a metal wire in an electric field the electrons in it will respond to the field and move along the wire. The field due to the displaced electrons tends to oppose the original field. If you do that with a field that changes with time the field drags the electrons up and down the wire. But that movement is a current. And a current flowing in a wire will encounter resistance and that transfers energy from the field and converts it to heat.

So a set of conductive wires like a polariser will tend to tend to convert the light energy into heat; that's absorption.

If the wires are mounted at right angles to the field the electrons are not moved so far and so there's a lot less absorption.

I think you have explained very clearly the electrical effects that take place when vertical rays meet the chains. I have no trouble with this explanation and I will assume that it is correct.

What it does not explain is the general effect it is supposed to have on all rays of the same direction, and even those which, I have to repeat myself, could easily slip through the gaps. What makes it impossible for those rays to go through?

Also, this explanation only takes care of one aspect of the dual nature of light. It is only valid for light as a wave. I think it would be very difficult to apply it to light as a particle. Unless, somehow, the gaps would only let electrons through with a certain polarization, if that is even a property that can be applied to electrons.

I would say that when light is viewed as a particle gaps have no way of differentiating between horizontal or vertical waves.

I look forward to your reply.

[Strange]

8 hours ago, Dalo said:

What it does not explain is the general effect it is supposed to have on all rays of the same direction, and even those which, I have to repeat myself, could easily slip through the gaps. What makes it impossible for those rays to go through?

It is because of the relative alignment of the electric field in the light wave with the molecules (John did not explicitly state this in this longer explanation but it was mentioned before). So those waves where the electric field oscillations are aligned with the molecules will be blocked, those at right angles will not (or much less so) and those in between will be affected proportional to the cosine of the angle.

8 hours ago, Dalo said:

Also, this explanation only takes care of one aspect of the dual nature of light. It is only valid for light as a wave. I think it would be very difficult to apply it to light as a particle. Unless, somehow, the gaps would only let electrons through with a certain polarization, if that is even a property that can be applied to electrons.

This is a very good question. The photon description and the wave description must give the same result, even if the mechanism is different. I believe this is related to photon spin and how that interest with the electrons in the molecular bonds but I don't understand the relationship between spin and polarisation (I don't know if spin aligned in a particular way is the same as being polarised in that direction, or if it is more complicated). So I look forward to seeing the answer.

[swansont]

10 hours ago, Dalo said:

I think you have explained very clearly the electrical effects that take place when vertical rays meet the chains. I have no trouble with this explanation and I will assume that it is correct.

What it does not explain is the general effect it is supposed to have on all rays of the same direction, and even those which, I have to repeat myself, could easily slip through the gaps. What makes it impossible for those rays to go through?

Also, this explanation only takes care of one aspect of the dual nature of light. It is only valid for light as a wave. I think it would be very difficult to apply it to light as a particle. Unless, somehow, the gaps would only let electrons through with a certain polarization, if that is even a property that can be applied to electrons.

I would say that when light is viewed as a particle gaps have no way of differentiating between horizontal or vertical waves.

I look forward to your reply.

Photons are not little BBs. They will interact with things over some area. Your mental model of "gaps" is flawed, but as you insist on using it, it would be that the gaps are small compared to the possible interaction area of the photons. There is no way to pass through the polarizer in a way that the light wouldn't interact with the polarizing material. See my analogy above about walking through a doorway.

[studiot]

3 hours ago, Strange said:

This is a very good question. The photon description and the wave description must give the same result, even if the mechanism is different. I believe this is related to photon spin and how that interest with the electrons in the molecular bonds but I don't understand the relationship between spin and polarisation (I don't know if spin aligned in a particular way is the same as being polarised in that direction, or if it is more complicated). So I look forward to seeing the answer.

 

Swansont has now said twice that photons do not act as classical point particles in these circumstances.
This is the key to understanding polarisation from a photon point of view.

You have to move the probabilistic quantum mechanics.
Then the explanation is the same as underlies all quantum effects.

Since this is Dalo's thread I have presented principle only, although the mathematics can be shown to achieve the same result as classical wave theory.

Fig 1

Shows a standard unpolarised transverse wave with the electric vectors ranged evenly around the propagation axis.

Fig 2

Shows the classical view of a vertically polarised wave. It has electric oscillations only in the vertical direction and that's it.

Fig 3

But in quantum mechanics all the other vectors still exist, however they are shown dashed..

That is because QM regards each as valid state. It is just that only one state is populated.
However in QM every state has a non zero probability of being populated.
Such a situation only becomes apparent when the light interacts with something, such as a slit.

Fig 4 Shows a vertical slit where the probability of transmission is 100%.

Fig 5 Shows a slit at 45^o, where classically the probability of transmission is zero, but in QM the probability is 50% half the photons move to the 45^o degrees polarisation level and half are absorbed. This also explains why the exist phtons are now polarised at 45^o

 

[Strange]

OK. So the amplitude of the wave at any given polarisation is related to the probability of a photon with that polarisation passing through. That makes sense.

Is there some relationship between the spin of the photon and polarisation? 

[studiot]

I don't think spin quantum numbers need be involved - do photons have them and aren't they discrete>

The circle of transverse vectors form a complete continuum of states around the axis.
It is the change from state to state that is quantum.

[Dalo]

3 hours ago, swansont said:

Photons are not little BBs. They will interact with things over some area. Your mental model of "gaps" is flawed, but as you insist on using it, it would be that the gaps are small compared to the possible interaction area of the photons. There is no way to pass through the polarizer in a way that the light wouldn't interact with the polarizing material. See my analogy above about walking through a doorway.

Concerning the argument of a long pole and a small door: the argument in itself is I think wrong because it already assumes that only poles of a certain orientation are available to go through the small opening. This argument is a very good image to explain how only certain rays/waves pass through, but not why only such rays/waves are available.

 

Concerning my mental model of a gap. I admit that I find it impossible to imagine a polarizer without gaps, and therefore very difficult to imagine how light could not go through those gaps.

The comparison with gratings is apparently still present, not only in my mind, but also in that of others in this discussion.

To make the dialog as productive as possible, allow me the following remarks.

1) I do not deny what my own eyes can see, the fact that when rotating a polarizer different effects are observed. In this regard the video's about the Malus experiment are very instructive.

2) I would very much appreciate an explanation of the material differences between a polarizer and a grating. For instance, in the videos mentioned, a polarizer is used with variable angles of polarization. It seems to me that such a polarizer will resemble more a grating than it does a Polaroid sheet with its permanent patterns.

3) Related to (2), how am I to understand the gaps in a polarizer?

4) I find it easier to think in terms of particles than waves, and I would appreciate it if an explication of polarization were given in those terms. Feel free to use as much math as you want for others, but please remember I am neither a mathematician nor a physicist, so I will need also verbal explanations and not only formulas.

Edited November 20, 2017 by Dalo

[swansont]

15 minutes ago, Dalo said:

Concerning the argument of a long pole and a small door: the argument in itself is I think wrong because it already assumes that only poles of a certain orientation are available to go through the small opening. This argument is a very good image to explain how only certain rays/waves pass through, but not why only such rays/waves are available.

1. Yes, such are the limitations of analogies. They only explain one thing, but then, their purpose is to explain one thing. Not everything.

2. Those are the limitations of living in a 3D world. Light is a transverse wave, so there are only two directions available to you for the polarization.

Quote

Concerning my mental model of a gap. I admit that I find it impossible to imagine a polarizer without gaps, and therefore very difficult to imagine how light could not go through those gaps.

Then you should work to remove these deficiencies in your understanding. Light passing between conducting plates can't exist if the wavelength is too long. If you could imagine yourself as the light, it doesn't "see" that there are gaps. Much like you don't worry about slipping between the gaps in a screen. There's no way you can fit without interacting, so it's as if the material was solid, as far as going through it is concerned.

Quote

 3) Related to (2), how am I to understand the gaps in a polarizer?

Stop thinking of them as gaps. The material is opaque to one polarization and transparent to the other.

Quote

4) I find it easier to think in terms of particles than waves, and I would appreciate it if an explication of polarization were given in those terms. Feel free to use as much math as you want for others, but please remember I am neither a mathematician nor a physicist, so I will need also verbal explanations and not only formulas.

As particles they are not points. They can interact over some range.

[Dalo]

1 minute ago, swansont said:

1. Yes, such are the limitations of analogies. They only explain one thing, but then, their purpose is to explain one thing. Not everything.

2. Those are the limitations of living in a 3D world. Light is a transverse wave, so there are only two directions available to you for the polarization.

Then you should work to remove these deficiencies in your understanding. Light passing between conducting plates can't exist if the wavelength is too long. If you could imagine yourself as the light, it doesn't "see" that there are gaps. Much like you don't worry about slipping between the gaps in a screen. There's no way you can fit without interacting, so it's as if the material was solid, as far as going through it is concerned.

Stop thinking of them as gaps

As particles they are not points. They can interact over some range.

The 'gaps" as explained by Land in his patent are certainly large enough to let light through.

[swansont]

3 minutes ago, Dalo said:

The 'gaps" as explained by Land in his patent are certainly large enough to let light through.

The word "gap" doesn't appear, not do I see any mention of sizes. Maybe you can narrow down what you are citing?

[Strange]

Just now, Dalo said:

Concerning the argument of a long pole and a small door: the argument in itself is I think wrong because it already assumes that only poles of a certain orientation are available to go through the small opening. This argument is a very good image to explain how only certain rays/waves pass through, but not why only such rays/waves are available.

All orientations exist before the polarising filter. (In other words, the light is unpolarised.) Only the orientations that can get through exist after the filter. That is why the light is polarised. 

5 minutes ago, Dalo said:

4) I find it easier to think in terms of particles than waves

The description above in terms of particles is quite good at avoiding one of the shortcomings of the "pole" analogy, which is that pole either goes through or it doesn't. The photons have a probability of passing through with a given polarisation.

[Dalo]

17 minutes ago, swansont said:

The word "gap" doesn't appear, not do I see any mention of sizes. Maybe you can narrow down what you are citing?

From the patent application:

"The die-members and the mass 10 in Fig. 1 may be deemed to show the parts greatly magnified as the die-members in practice are spaced apart a distance not much greater than the least dimension of the polarizing bodies"

 

In fact, it would seem that we were, or at least I was, all wrong about the polaroid sheets, at least in this version. There are gaps everywhere, and not only in one direction or the other. We are dealing with discrete crystals, also transparent, and not with continuous, opaque surfaces.

Edited November 20, 2017 by Dalo

[studiot]

Since the OP has not bothered to continue dscussion with me for my last several posts I can't see any point my continuing in this thread, despite the directed effort I put in.

This is his loss since if he had looked up Malus two mirror expreriment I had linked to, he would not have needed to ask some of his most recent questions.

I'm sorry I bothered.

[Dalo]

2 minutes ago, studiot said:

Since the OP has not bothered to continue dscussion with me for my last several posts I can't see any point my continuing in this thread, despite the directed effort I put in.

This is his loss since if he had looked up Malus two mirror expreriment I had linked to, he would not have needed to ask some of his most recent questions.

I'm sorry I bothered.

I am really sorry about that. I did refer to the Malus experiments from your link. I do not understand this reaction. If I missed something that you said that should have stopped all my questions, then please be so kind as to state it one last time.

2 hours ago, studiot said:

 

Swansont has now said twice that photons do not act as classical point particles in these circumstances.
This is the key to understanding polarisation from a photon point of view.

You have to move the probabilistic quantum mechanics.
Then the explanation is the same as underlies all quantum effects.

Since this is Dalo's thread I have presented principle only, although the mathematics can be shown to achieve the same result as classical wave theory.

Fig 1

Shows a standard unpolarised transverse wave with the electric vectors ranged evenly around the propagation axis.

Fig 2

Shows the classical view of a vertically polarised wave. It has electric oscillations only in the vertical direction and that's it.

Fig 3

But in quantum mechanics all the other vectors still exist, however they are shown dashed..

That is because QM regards each as valid state. It is just that only one state is populated.
However in QM every state has a non zero probability of being populated.
Such a situation only becomes apparent when the light interacts with something, such as a slit.

Fig 4 Shows a vertical slit where the probability of transmission is 100%.

Fig 5 Shows a slit at 45^o, where classically the probability of transmission is zero, but in QM the probability is 50% half the photons move to the 45^o degrees polarisation level and half are absorbed. This also explains why the exist phtons are now polarised at 45^o

 

If this is the post you wanted me to react to, then I must say I am not sure I understand it. Maybe it is evident to people with enough physical knowledge, but I do not really see any difference between what is supposed to be a wave, and what is supposed to be a particle.

The idea that a particle has probability of x% to pass through is completely alien to me. I understand statistical probabilities and am willing to accept the metaphorical use of the expression, but a single particle has, as far as I am concerned, no probability of going through, except if this probability concerns our own knowledge of it. Maybe it has nothing to do with the issue we are here discussing, but that reminds me of Schroedinger's cat.

[swansont]

1 hour ago, Dalo said:

From the patent application:

"The die-members and the mass 10 in Fig. 1 may be deemed to show the parts greatly magnified as the die-members in practice are spaced apart a distance not much greater than the least dimension of the polarizing bodies"

 

In fact, it would seem that we were, or at least I was, all wrong about the polaroid sheets, at least in this version. There are gaps everywhere, and not only in one direction or the other. We are dealing with discrete crystals, also transparent, and not with continuous, opaque surfaces.

How do you discern that these are gaps large enough to let light through?

Light gets through glass, even though it is made up of discrete molecules. The light just doesn't interact with the molecules. It's not fitting through gaps between them. 

The dimensions given are that the "least dimension" is about 600 micromicrons 
"crystals with a width of 600 ,up (micromicrons) and a longitudinal dimension of 1 to 2 ,a (microns)"

So that's 0.6 nm, which is much smaller than the wavelength of visible light. You must also consider that you don't have a single layer. The crystals will not line up throughout the thickness of the polarizer — they will overlap. 

You need to get the notion that light is fitting in through gaps out of your head. Light is incident on the material, which absorbs light of one polarization. It allows light of the orthogonal polarization through, because to that light it is transparent.

[Dalo]

31 minutes ago, swansont said:

So that's 0.6 nm, which is much smaller than the wavelength of visible light. You must also consider that you don't have a single layer. The crystals will not line up throughout the thickness of the polarizer — they will overlap. 

I will accept those figures, which means that light cannot "squeeze" between the gaps. In that, my mental model was wrong, just as you said.

Maybe you will help me ameliorate my mental models even more by explaining to me the case in which polarization takes place in what used to be called tourmalin, and which, just like Land's Polaroid sheets,is completely transparent. Because, I have also to admit that my mental model of "gaps" is necessarily complemented by a model of not-transparent, opaque, crystals or surfaces.

I see, now that I have read the patent application more carefully, that I was wrong. Just like tourmalin or calcite, the crystals are transparent. That makes the idea of absorption and transmission really obscure in my head.

 

Also, I have to insist on the necessity for me to understand how a variable polarizer is fabricated constructed. In fact, I am sure that if that became clear, we would probably conclude our discussion more swiftly.

Edited November 20, 2017 by Dalo