The Polarization Riddle

No idea, I will be interested in the solution myself....

John.

Please, someone answer this one!! I agree, it disagrees totally with what I thought was happening here.

As you all know, when light passes through two polarizing filters that are turned with thier long chain molecular stuctures perpendicular to each other, the light passing through both is blocked entirely due to the two step polarization. BUT, when you insert a third filter between the first two and turn it to an angle that is not perpendicular to the other filters, light passes through the first and is polarized to a particular plane. When that light then hits the filter in the middle the light is "twisted" to the pass through angle of the middle filter (and dimmed). When the light then hits the last filter, it is no longer perpendicular the angle at which the light would be blocked from the first filter and hence passes through (dimmed further). This is obvisously an oversimlified explanation but it provides enough detail to get the idea.

Keep in mind the picket fence analogy is only an analogy and is not wholly accurate, as I'm sure you're aware. Studying the wave theory of light in a non-analogous way opens the door to understanding what happens with the addition of a third filter. Also note, light is polarized to a plane perpendicular to the long chain molecular structure of the filter. This is analogous to a picket fence only allowing light perpendicular to the slats to pass through (no parallel). Hence, the innacuracy of the first analogy.

That's correct Guest, but what you're forgetting is that the poster said that the third filter is placed after the first two perdendicularly aligned filters...

So the third filter hasn't ANY light on it at all... as its all been absorbed / blocked by the first two 90* aligned polarising filters...

So how come??

I agree if the filter is put inbetween the first two but that's not what was stated...

John.

Hi Electroman, you wrote: "So the third filter hasn't ANY light on it at all... as its all been absorbed / blocked by the first two 90* aligned polarising filters..."

I'm a bit rusty on this, but I seem to remember that the +45° and -45° polarized components go through both the first two filters but cancels each other out. Now put a + or - 45° filter at the end of the chain and wella, you've got one of them getting through!

I maybe quite a bit off the mark! Hopefully Europium and Physicist can help us out...

Regards, Jorrie

Laser output is coherent light--already polarized with the "vibrations" of the photons all in the same axis. Here is at least one "incoherent" scenario, through the back door perhaps.

1. Filters block (absorb) light, instead of reflecting it, so this absorbed energy heats the filter.

2. Heating changes the physical and chemical properties of the filter, either temporarily or permanently--it eventually has to come to some form of equilibrium between energy absorbed from the incoming laser beam and energy released by thermal radiation or other means. At that equilibrium point it is probably not absorbing most of the incident light.

3. The human eye can perceive an incredibly wide range of light intensity, but is far less precise than instruments at measuring differences in intensity or transmission.

4. The described observation sounds a little like a magic act, in which the docent may have known what was happening and arranged the timing and placement of the polarizing filters so that the question that was asked mis-directed the attention of the people.

So--here's my "incoherent" explanation: After a certain number of seconds' exposure to the laser light beam, the polarizing filter loses its effectiveness. The placement of the first filter is the same orientation as the polarization of the laser. The second one is at right angles, blocking perhaps 90-99% of the light. The third filter is oriented the same as the laser and is placed there at about the time the second filter is known to become ineffective. The eye does not perceive the modest loss of light intensity as the laser light is transmitted through the filters.

5. Another possibility--as the filter heats up it rotates the angle of polarization of the laser beam so two crossed filters are in practice not crossed (?)

I'd be interested in seeing evidence that the laser light beam actually disappeared between the second and third filters. Someone out there with access to a He-Ne laser and a few filters--what do you see?

Regards--John M.

My name is Kevin, my friend posed this question and I am going to put this to the test. What I think is going on here is cancelation by slits. I bet if you rotated the third inserted filter the light would come and go all the same as it does with two filters. The third added to the two causes the choerent light to pass through the others albeit lesser power becuse its passing through in sort of an X or through the cracks so to speak. I am sure this is what is happening here. There is nothing at all to do with heat. If you have a hene that can heat a glass filter I would like to see it, it must be 20 feet long lol.

Schlieren photography and geological microscopes use a similar technique to produce images. The both start off with a beam of light that is roughly parallel then pass it through two polarizing filters set at right angles so that not light passes through. The image being studied is placed between the to filters.

What happens is that the first and last filters cancel out all the incident light and only allow the light that is distorted by the object being studied to pass. The result is that very subtle details of disturbances in the air and the crystalline structure of minerals are revealed as only the light effected by these subtle items can pass through to the viewer.

What I believe is happening in this case is something very similar to this. Any light that is undisturbed will be cancelled out by the first and last filters but the light that is disturbed by the middle filter can now be seen over the background light. One thing that you havn't mentioned is the ultimate intensity of the light leaving the system and I suspect it would be something like only 1% of the incident light.

What you have witnessed is universal expansion. The light as measured through our methods only appears to bend through the first and second filter...however you must now understand that only your perception of what happens to the light is confused by the third filter. All of the concentrated light passes through each filter and its waves realigned so this event is need of an explanation of our understanding which will require new methods of measurement. Just as the understanding of the earths rotation about an axis was measured by the movement of a pendulum.

Pete A

Universal expansion ????

'All of the concentrated light passes through each filter'.... Errrm what planet are you from?

John.

Electroman is keeping this discussion on track...

First, the laser was a small HeNe, probably no more than 1 or 2 mWs. So no heating there. Second, the third filter was not placed "between" the first two, but "after" the first two filters. Third, the docent had to rotate the third filter to get the light out, which was about 50% as intense as the original beam. Finally, when she removed the third filter, the light disappeared again.

She was not a shill. She was just some kid they'd given a script to, and the script did not call for a third filter. It was her own curiosity that got her into this and she really had no idea how to explain it.

Everybody, thanks for the input so far!!!

Interesting: there is a posabillity that the molecular structures act as a radio antenna. this could cause the light to cancel in the second filter and realigned in the 3rd. to test this it seems a diode laser pointer and a piece of paper could go a long way to proving it.

Huh????????

This makes sense if the polarizers are added in series moving closer to the laser.

Light from the HeNe laser is polarized at an angle determined by the brewster window inside the laser. The first polarizing filter P1 was placed not crossed with the plane of polarization, therefore letting some light through. She placed a second polarizer P2 in front of the first (between the laser and P1) and turned it so it was crossed with the polarized light from the laser. When she added a third polarizer P3 in front of the other two (between the laser and P2), the plane of polarization from the laser was changed. (The plane of polarization is set by the last polarizer it has gone through.) The last polarizer added changed the angle of polarization to not be crossed with the second polarizer. What got through the second polarizer was not blocked by the third.

Example to clear up this confusion:

Light from laser is polarized at 0 degrees. First polarizer P1 added at 30 degrees. Some light gets through. Second polarizer P2 added between first and laser at 90 degrees. Light from laser is blocked. Third polarizer P3 added between laser and second at 45 degrees. Light from laser passes P3 (attenuated), now polarized at 45 deg. It passes through P2 (attenuated) now polarized at 90 degrees and finally through P1 (again attenuated) now polarized at 30 degrees.

I'm puzzled. Your post seems to suggest that the polarizing filter, if it is not crossed--at 90 degrees--to the previous filter, acts to change the polarization angle of the light transmitted through it. If this is what you meant, I would disagree. Although there are many materials which rotate the angle of polarization, as the light passes through them, I believe that standard polarization filters don't. John M.

Once again Guest, you are stating that the third polariser is added between the two 90* polarised filters...

Its not! the third filter is added at the end of the filters, not between them.

Hey John M. I'm a John M. too snap!

Yes, but the big question is, how does this effect my aviators?

The trick is that the light starts out polarized out of the laser. The last polarizer added is between the laser and the second polarizer added. It messes up the cancellation between the laser and that second polarizer.

When you pass polarized light through a polarizer, the light is then polarized in the plane of the polarizer and the amplitude is (sans losses) the component in the polarizer direction. If it is 90 degrees, then the component is zero.

As the White Knight said, "Curiouser and curiouser!"

Here's a quick experiment I just tried; It's kind of a reverse from the original situation... So please don't let this detract you from the original question.

Anyway, As you might know, the light from a digital LCD display is polarized. So, I took the Polaroid filter from my camera, held it up before the LCD display, then rotated the two filter elements until no light was passing through. Next, I put one lens of a pair Polaroid sunglasses between the screen and the camera filter that I was holding up to my eye. As I rotated the sunglass lens, light began to make it through the camera filter!!! Again, its maximum was about 50% of the unfiltered screen!!! How about that!

Also, jmueller is correct. Being a laser nut, I have a lot of non-linear optical devices that can separate and recombine light into its various electrical components ( S-polarization, P-polarization, and elliptical to circular polarizations). However, these devices are seldom made of "just glass" or Polaroid filter material. And the action of these devices are very apparent: you shoot a single laser beam in one side, and two beams (S-p and P-p) come out the other side. Stuff like that.

Again, it's the curious behaviour of these simple filters I find strange. As I was taught by my Science teacher: "filters just block light, other than that, they do not affect the light." I think he lied.

Hi all,

As far as i know the light in a laser ( light amplication by stimulated emission of radiation ) is made to bounce back and forth within the laser body until it has enough energy to burst through the laser window.

Now correct me if i'm wrong but a visible laser beam is a focused beam of light?

The light photons emerging are travelling like "packets" of information in one direction but the packets may be at angles to one another? These packets are like "wavelets" or a small bit of a waveform?

Imagine three cars going down a road; one is standard, one is rolling along with wheels on its roof and another travelling on 2 wheels at 45 degrees ( like you see in a stunt movie ). Is this not how light photons travel?

Is this a new-ish light theory? >>

The 45 degree ones get through the "holes of the mesh" created by a filter like ||| and one like = . If the 3rd filter is then at this angle or perpendicular to it, would it not seem plausible that the light seen is the concentrated 45 degree photon packets?

What i dont get is, is the human eye with it's retinal rod and cone cells sometimes insensitive to light wavelets travelling in a certain direction?

How come light is not seen after the 2nd filter? I'm not a physicist or a biologist, i thought i'd add my thoughts here though.

Regards,

Dr M.

Hi folks,

I was playing with some polarizing filters in an attempt to get some images for this thread and discovered something else about polarizing filters that I didn't know, have no idea why and has confused me no end..

The following image is of the screen of my laptop which is polarized vertically. As expected If I put a polarizing filter so that it is aligned with the screen light is not blocked.

I thought I was going mad because after putting down the filters and picking the back up one of the filters now blocked the light when aligned vertically. Some investigation revealed that if you flip the lenses over so that the light passes through them in the opposite direction the polarizing effect is rotated thorough 90°.

As you can see in the second image I have not rotated the filter at all, I have just flipped it round so that the light now passes through it in the opposite direction. You can clearly see that the filter is now blocking all the light from the screen.

I have absolutely no idea why turning the filter over would change the orientation of the polarizing effect by 90°. I am now even more confused about the properties of polarizing filters that I was before starting the experiment. designed to shed light on the properties of polarizing filters.

OK guys HELP, what the smeg is going on!

See?! I'm telling you, all the science teachers are messing with us. It's like the Manchurian Candidate, only with Polaroid!!!

One thing you might check is the rotational angle of the filter. I tried your experiment and got the same results. However, if I was really, really (and I mean really) careful not to rotate the filter as I flipped it, the light reduction was about the same. Even a very slight rotation makes a lot of difference! Try it out, and let me know what happens.

Thanks!!!

Oh GREAT!! I'm glad we have a few more brains working on this!!

I know that using polarising filter you have to have it the correct way round, that's why when you buy them they will have a label on saying which side is which...

But as to why?.... I'm totally in the (non-polarised) dark!!

I think I will go to my lab now dig out a laser and some polarised filters to give it a try!! Seeing as nobody has an answer at least I can return with some further confusion!!

John.

OK, just to make thing "possibly" easier, I've been thinking that circular polarization may be involved. However, I don't think you can achieve circular polarization with a filter... I think it takes some sort of wave-plate or non-linear optical component. Any ideas?

Your laptop screen is probably polarized at 45 deg.

Hi Guest,

In post #22 you stated,

Your laptop screen is probably polarized at 45 deg.

Initially I thought this didn't make any sense but upon pondering your statement further I realized that this is indeed what was happening.

I took the lenses and aligned the shaft on the lenses so that it was at a 45° rather than horizontal when all the light was blocked.. The end result was that the rotation of the filters around the shaft no longer causes the polarization to rotate through 90° when the filter is flipped.

Good thinking Guest, have you thought of joining the CR4 community, I am sure we would all benefit greatly from your input.

Ok that means that there is no rotation of the polarizing when the filter is reversed so what about the rest of the conundrum.

OK, I think there are a couple points of confusion about vermin's initial post.

1. "In front of" I think we all agree means closer to the viewer. The viewer is not looking through the polarizers at the laser (hopefully) but is standing behind the laser looking at a screen. Therefore, when he says "then she places a second Polaroid filter in front of the first" it is between the laser and the first polarizer.

2. The plane of polarization of the light is determined by the last polarizer it goes through. It is like wiggling a rope through a picket fence. Leaving the fence, the rope wiggles in line with the slot in the fence.

3. The light from the laser is already polarized (by the brewster window inside).

Here are my pictures:

Laser and polarizer P1, P1 not cross polarized with Laser light, light gets to screen

LASER =========================== P1 ======= SCREEN

Add P2, rotate P2 to extinguish light (cross polarized with laser). Light stops at P2:

LASER =================== P2 ______ P1 _____ SCREEN

Add P3 (P3 changes polarization reaching P2 -- see 2. above). P2 now passes part of light, P1 passes part which reaches screen.

LASER ========= P3 ======= P2 ====== P1 ==== SCREEN

Hope this makes sense.

Ohhh! I thought that point had been cleared up earlier. My picture was the light source, then two filters totally blocking the light. Then a third filter added after the first two. You're saying the third filter is between the source and the other two filters. Totally different kettle of fish. I think vermin better clarify this one for us before I go and kick my science teachers headstone over!

Nope! It happened the other way around. To make this clear, please see the diagram...

I will try this out this weekend, and will be very surprised if it works in this order. Is there a possibility of a simpler explanation, for example, that she disturbed the alignment of one of the other polarizers in adding P3?

Hi Guest, I would share your surprise if it works! I was thinking along the same lines, perhaps even a "fastness-of-hand" trick to disturb a filter's rotation.

One problem of testing it is that we do not have enough info - i.e., is the laser polarized and if so, how (maybe circular?).

With normal unpolarized light, I'm pretty sure it can only work if the third filter is inserted between the first two. I think not even Shrodinger's equation of quantum probabilities will create light from zero input!

Regards, Jorrie

Oops, I goofed when I wrote: "With normal unpolarized light, I'm pretty sure it can only work if the third filter is inserted between the first two. I think not even Shrodinger's equation of quantum probabilities will create light from zero input!"

I'm now convinced that it was Shrodinger's half-dead-half-alive cat, in its superposition of states, that tweaked a filter while no-one was watching!

Jorrie

If P1 is a linear polarizer, whatever goes in should come out linearly polarized with some amplitude (zero if the input is linearly polarized at 90 degrees to P1). So the light source shouldn't matter at this point, as long as it's not polarized crosswise with P1. If P2 is a linear polarizer and is turned to block the light, it is oriented 90 degrees from P1. But when the light is blocked, it's blocked.

I'm still thinking that the alignment got disturbed. The null point is very sensitive and the laser beam is very bright. It wouldn't take much.

Anything that looks wierd should be repeated a few times to make sure.

If you read my earlier post 49 Guest this is what my conclusion was with my experiments...

John.

Oh yeah. Great minds and all that.

OK, here's the solution: The young lady was having us on, and right now she is reading all these theories and having a good laugh.

Hope this helps... The filters fit into slots in a wooden box. The box had NO top so that the filters could be inserted one-at-a-time, and then rotated. Rather than trying to describe this any further, please see this diagram...

Ok.. Now I am seeing how this can drive one completely nuts, after my program ends on TV I am doing this experiment! My first comment is now polarized and blackened. plik..

That expains why my laptop looks so much better with my head tilteed to one side while wearing my sun glasses.

10,000 people are now looking at there PC's wearing shades with their heads on one side. This is how epidemics get started.

What interests me is that the incident was true, yet no specialist (physicist, etc.) has popped in to tell us how it was done!!!

It worries me too, I had a quick look on the internet to see if there was anything that talked about polarizing filters in more depth that might shed some light on what is going on be as yet I havn't found anything.

I havn't got a laser to work with but I do have a TV with a LCD display and some polarizing filters. I plan to do some experiments shortly and I will post the images and results here.

That's what's fascinating me with this one too. Usually this sort of thread runs for a while and then some bloke with an unlikely name like Einstein or guest pops up with a relatively rational explanation. This time, despite some interesting discussion, I've yet to see a credible explanation as to how light that is apparently totally blocked by the criss crossing picket fence can be brought back.

The only credible explanation was along the lines of cancelation where the third filter somehow straightens out a cancelation effect relating to 45 degree light. If this sounds vague it's because I haven't a clue and I wish someone would explain!!

This clear up something for me. I was wondering why Lieutenant Caine on CSI Miami always wears sunglasses and tilts his head to the side.

And so the polarization virus claims another victim...............

think simple math

1 = 1

1 -1 = 0

90 degree phase shift + 90 degree phase shift = 180 degree phase shift

1 -1 + 1 = 1

90 degree phase shift + 90 degree phase shift + 90 degree = 270 deg

The light is not blocked but polarized. 180 degree phase shift = the cancelation of visible light or interferance. The light is still there just not visible

The third simply shifts it back into visible phase.

The laser light is not present in just the beam, it is reflected throughout the room , being absorbed and re-radiated on all objects. The fact that you can see it proves this point, some has radiated towards your eyes. The whole room is flooded with the original light frequency and phase.

The beam is phase shifted by the first filter, and reradiated throughout the whole room. Thus you add the two signals

The second filter also shifts the phase and is radiated. This is 180 degrees out of phase with the original signal, virtually cancelling the phase angle of the majority of the light. The light energy is still there. (just it has been virtually changed to DC from AC by the cancelling of phases) The third filter retransmitts the light back as AC at the original wave lenght.

This exhibit was out on the main floor of the Exploratorium. And that's a REALLY, REALLY, REALLY big room!!!

Hi techno, you said: "The light is not blocked but polarized. 180 degree phase shift = the cancellation of visible light or interference. The light is still there just not visible

The third simply shifts it back into visible phase."

I'm afraid that polarizing filters do not work on phase shifts. So your 'simple math' of 1 -1 + 1 does not work either.

Unpolarized light that has passed through a horizontally polarizing filter has 50% of the original intensity at horizontal polarization, 35% of the original intensity at 45° and 0% of the original intensity at vertical polarization. In between those angles it varies according to a simple trigonometric law. So most of the light is still there, just differently polarized.

I believe this riddle is a quantum-mechanical problem, where Shrodinger's wave equation comes into play. I'm just not sure exactly how!

Jorrie

That is my point.

The filters are obvious not perfect and cannot controll all the light, however

The original wave front is vertical polarity direction is forward ---|

The filter does a phase shift so the wave front is pointing , say down, (horizontal) |

= 90 degree phase shift. The fact that only 50% makes it is could be in the efficiency or the fact some light is radiated on all angles between the original and the final wave front and some may overshoot

The second filter results in the wave front shifting another 90 degrees |---- which is vertical polarity but 180 degrees out of phase with the original

The third filter would have the same effect as the first and the output would be ­­, say up­ |

The same technique is used in noise cancellation in automobiles

"believe this riddle is a quantum-mechanical problem, where Shrodinger's wave equation comes into play. I'm just not sure exactly how!"

This equation is part of the basic understanding of how light works. If an atom absorbs the light energy, the atom can:

1. keep all the energy,
2. Convert the energy to heat and radiate heat
3. Reradiate the light.
4. or any combination of these

If an object keeps all the light, the object will appear black in color

If the object reradiates all the light immediately, the object will appear (white)

If the object keeps some of the light: the rate of eradiation will determine the color of the object. (Different wavelength, frequency)

I'm thinking like you, Jorrie. The light is supposed to be screened out, and not phase shifted. Please see my diagram that I added to this thread. Thanks!

"I'm afraid that polarizing filters do not work on phase shifts"

Unless light passes through undeterred, there is always a phase shift.

The most simplest way to to think of a phase shift, is a time delay

This would make some weird sense, wave-like interference sense

As I said in a earlier post I havn't got a laser but by using a LCD (Liquid Cristal Display) television and I have been able to get something similar to but no exactly the same, as the experiment described by Vermin

I think a lot of the confusion is caused by trying to describe a complex optical problem with words so I will refrain from trying to describe what I have found for the time being. I intend to try and take some pictures so hang in there folks, give me a half a day or so and I will have some pictures for you to look at.

"And a camel is a horse designed by committee!"

Masu, did you see my diagram?

You rotten sod Vermin that was going to be my next catch phrase when this one wears out. Now I need to think harder and after the headache this thread is causing that's not going to be easy.

On the more interesting side I think I know what is going on with the polarizing filters and have the photographs as well so stay tuned. I am writing it up now and will post it in a couple of hours.

Please, stick with plan A. Just because someone's anticipated your next progression, that's no need to abandon it.

I'm finally adding my favourite tag to my posts. Trouble is I haven't anything to progress to because this one encompasses my entire philosophy. Never know though, time will tell!

Well, the mystery deepens!

I'm sure there is trick involved with this, yesterday I sat at my lab bench with a led laser and three neutral polarized filters...

Using them as described above, with filter three added after the first two, there is no way the laser light can be made to return, as suspected on here.

With the third filter reversed though and placed between the first two filters the light can be made to come back, once again as stated on here...

So I am beginning to think there was some sleight of hand involved in this trick....

John.

I should add to my last post that it would have been the same no matter what the light source was.

I did wonder if a laser was specified due to its highly visible light? As only a minute 'adjustment' mis-aligning the first two filters was sufficient to allow a highly visible beam through??

John?

One thing to remember is that a laser diode's output is really messy! Not so with a nice, TEM 00 HeNe beam. The light just spews out and has to go through some optical munching to even get it parallel. Most laser diodes are to some degree elliptically polarized, which means the light beams electrical fields are all out of step. Believe me... I'm in the process of bring on line a 50 Watt laser diode (diode array). Aside from the current draw, the hardest thing to get right is to get the damn beam to collimate!

Unfortunately I do not have a laser so what I have done is to use the screen of a LCD TV. Whilst this is not identical to the experiment as described by vermin I believe we can get a good idea of what is hapening.

The two polarizing filters are aligned so that the polarization is in the same direction as the mounting rod. For reference purposes 0° is vertically up and angles are measured from the vertical in a clockwise direction as viewed from the camera.

This first image shows the filters aligned at 90° and 270°. As you can see there is little to no attenuation of the light from the screen.

This image shows the tro filters so that the polarizing is at 0° and shows that there is little to no light passing from the screen through either of the filters.

This image show that approximately 50% of the light passes through the filters when they are aligned at angles of 135° and 215°.

This image shows the filters at 45° and 315° and again the light from the screen is attenuated by roughly 50%

From this we can conclude that the light from the screen is polarized horizontally or at 90°/270° and that when the filters are placed at 0° or 180° they completely block the polarized light from the screen.

The next part of the experiment consisted of placing a filter between the screen and the camera with the polarizing at 0° so that no light would pass from the screen to the camera. A second filter is thne added between the first filter and the camera and the effect of varying the polarizing angle of the second filter monitored.

This image shows both filters aligned at 0° and as you can see no light passes through both filters.

This picture shows the first filter at 0° and the second at 90° and shows that no light passes through both filters but the light from the screen that bypasses the first filter is not blocked by the second.

This image shows the first filter at 0° and the second at 135° and again no light passes through both filters. The light passing through only the second flter is now attenuated by 50%

And finally the last image in this first part of the experiment shows the first filter at 0° and the second at 180° and again no light passes through both filters.

As you can clearly see from the photographs with the first lens set at right angles to the light source no alignment of the second filter will allow light to pass through both filters to the camera.

This raises the question what is going on in the experiment to allow light to pass so I experimented a litter more and fount the following.

I set the second filter (the one closest to the camera) at 0° so that it blocked all the light from the screen and then rotated the first filter (the one closest to the screen) and recorded the results as follows.

We have already see what happens with both lenses at 0° so the first image in this part of the experiment shows the first filter a 45° and second at 0°. The light that passes directly from the screen to the second filter is blocked and about 50% of the light that passes through both filters reaches the camera.

With the second filter at 0 and the first at 90 no light is seen to pass through either filter.

Second filter at 0 and first at 135 allows about 50% of the light that passes through both filters to pass through but the light that passes only through the second filter is blocked.

With the first at 180° and the second at 0° no light passes at all.

This shows the first filter at 270° and the second at 0° and again no light passes. Note however that you can see that the light is passing through the first filter and is being blocked when it reaches the second filter.

This final picture shows the first filter at 315° and the second at 0° and again about 50% of the light passes.

At this point I will not draw any conclusions. I am going to think about the results I have seen so far and see if there is a way to interpret them in a way that would explain the results as described by.

Has this given anybody any dieas yet?

Call the polarizer closest to the screen P1 and the one nearest the camera P2.

If P1 is crossed with the polarization angle from the LCD, nothing you do to P2 will bring the light back after it is blocked.

In the last set, P1 repolarizes the light to whatever angle you hold it. The light transmission between two polarizers is (sans losses) cos^2 (theta) where theta is the angle difference. There are two factors: Angle between the LCD and P1 (theta1) and angle between P1 and P2 (theta2). The transmission through both is cos^2(theta1) x cos^2(theta2), where theta2 = 90 deg - theta1 (P2 is crossed with LCD). Maximum transmission will be at 45 degrees = (1/2)*(1/2) = 1/4.

To understand what is happening, you have to understand wave patterns, and interference patterns.

Light is absorbed and delayed by the filters

The time delay (phase shift) creates a new light wave out of synch with the original light

You always see the sum of all the light in the room

The sum of the two waves creates interference patterns, thus a dimming of the total light,

If you delay the light so it is exactly 180 degrees out of phase with the original light wave, the sum of the waves is zero, however, the light is still there-just 180 degree opposing waves

When you create interference, the sum of the two waves may create some dark spots and some brighter spots.

The energy of two waves in synch is accumulative

Hi techno, you wrote: "You always see the sum of all the light in the room

The sum of the two waves creates interference patterns, thus a dimming of the total light,

If you delay the light so it is exactly 180 degrees out of phase with the original light wave, the sum of the waves is zero, however, the light is still there-just 180 degree opposing waves

When you create interference, the sum of the two waves may create some dark spots and some brighter spots. The energy of two waves in synch is accumulative"

What you say above may be true in general optics. When it comes to polarized filters, this has no bearing on the results obtained so far. So please read Masu and electroman's results and think about what you wrote above...

Regards, Jorrie

Hi masu, impressive set of results!

You wrote: "At this point I will not draw any conclusions. I am going to think about the results I have seen so far and see if there is a way to interpret them in a way that would explain the results as described by.

Has this given anybody any ideas yet?"

I think your experiment confirms what Electroman found in post #47. Your TV screen acts as the first filter, horizontally polarized. When you place a vertically polarizing filter between camera and screen, all light is blocked, as expected. Your second filter between the first filter and the camera acts as the "third filter" of the original experiment. Whichever way you turn it, no light passes through the combination.

Your "Second filter at 0 and first at 135 allows about 50% of the light that passes through both filters to pass through but the light that passes only through the second filter is blocked." is equivalent to placing a third filter between the two filters of the original experiment at an angle: as expected, light passes through. This is the stock-standard lab experiment shown to students.

So, the original experiment described in the OP still seems like a hoax!

Regards, Jorrie

try one at 315 and the other at 45

If as someone said the laser light was already polarized, this would only be the first part of the experiment and another filter is required

Masu,

YES!!!!! I think you're finally getting some place!!! You've made me realize that the point-of-view may be getting rotated here! In other words, as in my diagram, the viewer is looking from behind the laser and viewing reflected light from the screen (I think it was a piece of paper). However, what if the viewer stands behind the screen to view the light coming directly from the laser (instead of its reflection). In that case, the point-of-view may act to rearrange the polarization angles. In effect, in your experiment, your LCD screen is "the laser" coming right at you, and your eye (or camera) is the "screen. " Then P1 -> P2 -> P3 effectively becomes P3 -> P2 -> P1 !!!

I think I can smell something cooking here... Hope it's not my last few neurons.

The only thing I would like to see you do is to add a circular polarizer to the system and see what happens then. Like I said about the presentation, I can't rule out that the docent had not accidentally picked up an "odd" polarizing filter. There's a lot of exotic stuff laying around at the Exploratorium. I know that certain anti-glare screens use a combination of linear and circular polarizers to get rid of reflected light and glare.

This is driving me nuts...

I finally sent an email to the Exploratorium. I'll let you know if I hear anything.

If he still works there, ask for Larry Shaw. He's Mister Physics (or was).

My first reading of this question was that it sounded like a hoax, and it seems like this may be the conclusion. I want to offer a possible explanation for a way something like this might of been observed although its a long shot. The polarization angle of some HeNe lasers rotate very slowly. I have observed this by putting a polarizing filter in front of one and seen the light slowly pulse on and off. Perhaps the polarization angle of this laser was rotating, and the girl had the filters set up so light would pass through them all. She then unknowingly placed and removed the third filter into the equation at the right moment so that it appeared it was causing the light to pass. I know this sounds like you would quickly realize what was going on, but to an untrained person who was playing I think this is a possible mistake.

Good point. I thought the polarization was fixed, but did a little research, and the cheaper ones can vary over time. I have an old HeNe at home that I might fire up this weekend and see what the polarization does.

From Paul Doherty Senior Staff Scientist at the Exploratorium. Guess I'll have to work harder training our Explainers. So here is what she did: Take a laser (any light source will work even a flashlight) Put it through a polarizer, the light that comes through the polarizer is linearly polarized paralel to the polarization axis of the polariaer (which is perpendicular to the long direction of the polymer molecules but that's another story) Now put this polarized light through a second polarizer rotated with ists polarization axis at 90 degrees to the first one and almost no light comes through. No component of the first polarization is parallel to the alignment of the second polarizer. Now put a third polariser BETWEEN the original two and rotate it until its polarization axis is 45 deegrees with respect to the other two. There is a component of the first polarization that is parallel to the middle polarizer, so that 1/2 of the light gets through this first pair (cos45°)^2 and the light that exits the BETWEEN polarize is now polarized parallel to its polarization axis. This light then goes through the third polarizer with a 45° angle between the polarization axis so once again a component of polarization gets through and (cos45°)^2 or 1/2 also gets through. So in the end 1/2 * 1/2 = 1/4 of the light gets through I hope this helps Paul D

Oh thank Gawd for that.... I thought we would never get to the bottom of this one...

So the third filter wasn't at the end of the filters is was put inbetween the first and second...

Now that makes sense!!

John.

I could have sworn that she put the third filter at the end, but I'm willing to concede at this point that I may have remembered it wrong. These days, it's a little harder for me to get the be-be in the Bear's eye; AKA my one working neuron.

However, I must congratulate all of you on really taking on this question and sussing it out among yourselves. You came up with great creative ideas. I think that this proves that the system (the CR4 community) really works!!! Special thanks to Masu and Electroman, you guys really hit this hard. Thanks for your time. And see, we didn't need an optical physicist after all!

Well, I think it's time for Vermin to crawl back under his rock. Until next time, "SkweeeeeeeeeeeK!"

Actually I have found the whole process very instructive and I have learnt a considerable amount about polarizing filters. I was under the impression that polarizing filters would only allow the light that is oscillating in a particular plane to pass through but it is clear, from the results we have seen, it is far more complex than that. I know what was happening it the in the experiments that you described and that I performed but as yet I am not 100% sure on the physics behind the results. I think I need to do some more reading about what and how polarizing filters work. Any links to sites that have information on the principals involved in polarizing filters would be greatly appreciated.

Actually, I've been looking for a good "heavy" physics book on the subject. As I said in one of my postings, I'm messing with laser diodes; and the light coming from them is a real nightmare for the layman. Instead of individual photons, you have to sort of think of the beam (many photons) as an entity in itself. Then, you get into these weird situations where the electric field is not in phase as with standard Lasers. Instead, the electric field of the beam moves in and out of phase with itself; causing a "slow" axis and a "fast" axis. It usually takes wave plates or special lenses to get it all back together. Mind you, you can't even think about collimating the beam from a diode laser until you conquer the polarization problems... A lot of this arises from the fact that most diode lasers are really microscopic piles of many laser diodes; and guest what, because of this the raw light from a laser diode isn't even coherent!!! You even have to force the individual wavelets into coherency! Really sucks!

Anyway, if I find a good source for polarization info, I'll be sure to tell you. Please do the same for me.

Just wondering if this is sort of like thinking about an atom as a miniature solar system. The concept goes only so far until it starts causing problems. Sooner or later, you have to learn about quantum physics to understand what's going on with the electrons. I think we're both at somewhat the same point with polarization.

PS: Again, thanks for you participation in this. It was fun, and you really helped! ...Did I say fun?! Damn! Well, I guess that makes me a geek!!!