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This comes from one of my physics course at university (many years ago!), I hope I recall it right.

We were studying optics, and were given some polarizer filters to experiment. They were disc- shaped (like a photographic filter). I stacked two polarizers, face to face, and I verified that, when rotating one of them, a particular orientation (and its 180 degrees rotation) blocked the light. That seemed to make perfect sense to me, because I pictured the polarizer as a wire grid (conceptually like this image)

enter image description here

that only let pass light in a given polarization direction: then, adding a second polarizer to the other behind had no effect when the direction of the wires coincided, and result in high light blocking if they were orthogonal.

We were all satisfied with this explanation... but then I tried to reverse the front filter (I mean, I turned it face down, always keeping it in front of the other, and keeping its "orientation"). To my surprise, keeping the orientation did not keep the block-pass behaviour. Say, if originally the filters were at the "let it pass" position (paralells wires, in my mental picture), putting the filter face down (which would let the imaginary wires in the same orientation) resulted in light blocking, and I had to turned it 90 degrees to let pass the light. This behaviuor puzzled me a little, as also puzzled my teaching assistant (who didn't come with any explanation). I just assumed that my "grid wire" picture was not very apt, but that was all (not enough scientific spirit on my part, I guess).

Can anybody make some sense of this?

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""I joined two of them, face to face, and -fixing the filter behind - "" This is irritating : "two of them" and a filter? What is this filter? A third one? –  Georg Jun 5 '11 at 14:53
    
Sorry for my english, feel free to edit to make clearer the meaning. –  leonbloy Jun 5 '11 at 16:36
    
Still unclear You write of two filters face to face, then filters and then "the filter". This is not a problem of English. And because that polazizers were circular, what do the angles You mention mean? –  Georg Jun 5 '11 at 16:48
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2 Answers 2

What you are seeing is likely the result of the the light becoming polarized by virtue of the fact that it is now entering the surface of the tilted glass filter at something other than a 90° angle. When light passes into a refractive medium at 90° all polarizations of the incident beam are treated equally. But when the beam of light enters at an angle to the normal to the surface, the reflected portion starts to favor those photons whose electric field vector is parallel to the surface. As a result the transmitted light is deficient in those polarizations and becomes polarized at right angles to the reflected portion.

So now you have two polarizing grids competing with a polarization component due to the tilted filter. Strange things happen.

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Where does leonbloy mention "the tilted glass filter "? –  Georg Jun 5 '11 at 16:50
    
"... but then I tried to reverse the front filter (I mean, I turned it face down, always keeping it in front of the other, and keeping its "orientation")." I interpreted this statement as that he was tilting one of the filters in the process of flipping it over. I have done it myself and I remember being puzzled as well. –  Bill Slugg Jun 5 '11 at 17:59
    
And I thought You mention a polarisation filter made from glass sheet(s) at brewster angle. –  Georg Jun 5 '11 at 18:37
    
You are correct. The filters also act as Brewster polarizers. The filters that the man had were simple camera polarizers. Usually two sheets of glass bonded to a core of stretched plastic. You can easily replicate them by stretching food wrap such as Saran Wrap (polyvinylidene chloride). The same filters can polarize also by virtue of the fact that they employ sheets of glass that reflect. –  Bill Slugg Jun 5 '11 at 21:24
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Your "grid wire" picture is essentially correct. One possible explanation for the weird observation is that the polarization of the filter was not aligned with the edges of the piece of polarizer. That is, you imagined that the wires in the polarizer run parallel to the edge of the polarizer, but they were actually tilted.

If the wires run at a 45-degree angle to the edge, then turning the polarizer around will make the wires go from tilting up at 45 degrees to tilting down at 45 degrees - an effective 90 degree rotation overall. This would explain why it blocked light after being flipped.

You could achieve the same effect with the more typical polarizers in your picture by rotating around the axis of the diagonal, rather than around an axis parallel to a side.

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btw, if this was the case, @leonbloy could have detected this by trying to gauge the real orientation of the polarizers by making some masurements first on ech filter, and using several angles around the "labeled" direction of the filter, measuring intensity of light passed, and comparing the maxima of the intensity versus the deviation angle. –  lurscher Jun 5 '11 at 15:10
    
amazing explanation. but what kind of diabolical people would sell you a filter like that? –  Marty Green Jun 5 '11 at 15:17
    
My guess is that students twisted the filter relative to its alignment marks. Alternatively, it might have had not marks so in guessing the alignment, the students were off by 45 degrees. (I.e. if polarization is not marked on the filter, then the only way to figure it out is by the experiment described by the OP. –  Carl Brannen Jun 10 '11 at 4:17
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