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# Polarizing Filter Mystery!

#### Benm

##### Well-known member
I'm not fully certain, but could it be that this filter actually has 2 different polarizing elements in it?

One being circular, and the other linear?

If that were the case you could block out the LCD monitor light fully when the linear side is facing it at the right angle, but not if the circular side was.

Another issue is that it seems to go almost completely black to randomly polarized light, which you would not expect from 1 circular + 1 linear (it should block 75% or so).

Last resolution could be that it as more than 2 polarizors, perhaps 2 of which could be on one rotateable element. For example a circular one, and a linear one in 1 of the glass pieces, and then another linear one the the other piece. That would explain why it can both block light almost completely, but only in one orientation.

This is all a bit confusing, but you can do a very interesting experiment with 3 linear filters: position them like a venn diagram, and you actually get a brighter piece in the middle and 3 dark pieces around that, and 3 50% pieces where light only passes one filter.

#### Gabe

##### Well-known member
I haven't learned exactly how circular polarizers are structured quite yet, but I believe you're correct and we've gotten to the bottom of this. This filter must consist of a rotatable linear polarizer in the front, and behind that a circular polarizer. And these circular polarizers consist of a linear polarizer in front of a quarter wave plate. When the two linear filters are aligned and already polarized light passes through the filter from the front, the light will either be passed or blocked depending on the orientation of the two linear filters (acting as one linear filter) with respect to the already polarized light. The quarter wave plate at the very back has no effect. When the filter is reversed, the quarter wave plate converts the previously polarized light from the lcd screen to circularly polarized light, which then can't be blocked in full by the parallel linear polarizers in any orientation.
Thanks so much guys for clearing this up and ripping me off to circular polarizers.

D

#### Deleted member 16589

##### Guest
Wow that's weird never seen that happen. There is another weird phenomenon in which if you put a third filter at an angle between the 2 it will cancel out the effect.

#### Singlemode Laser

##### Well-known member
Its not 2 polarizers but one polarizer and one wave plate. One "filters" polarization and the second one creates circular polarized light from linear one.

@lazerman:

There is nothing weird in the effect of the third polarizer sandwiched between the two orthogonal (to each other) ones.

Singlemode

#### paul1598419

##### Well-known member
Gabe, a quarter wave plate IS a circular polarizer by definition. Two quarter wave plates make a half wave plate which changes the direction of the plane polarizer. If you have light go through a quarter wave plate, it is circularized and if it goes through another, it is plane polarized again, only shifted in direction.

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#### Singlemode Laser

##### Well-known member
I think there is some misunderstanding in the wording here. While both terms, wave plate and polarizer can be used for an element that changes the polarization of light, I think it makes things more clearly if one uses wave plate or polarization retarder for this and polarizer for elements that filter tne polarization state.

Singlemode

#### paul1598419

##### Well-known member
Yes, that is the correct terminology. A wave plate is a polarization retarder, while the plane polarizing filters are in fact a filter that only allows the light in one plane to pass unobstructed. I was trying to explain a concept about wave plates to Gabe without going into too much detail. The fact that wave plates are wavelength dependent and also are made from birefringenent materials, so the axis they are cut at and the thickness of the retarder are important factors when using one with a single wavelength of light.