Flipping a polarizer 180deg. changes polarization

[i_amitabh]

If you have played around with polarizers you might have noticed that flipping a polarizer by 180^o about the vertical axis has the same result as rotating it by 90^o about the horizontal axis[i.e. flipping so that the face of the polarizer which was facing away now faces you has the same effect as rotation by 90^o]

I have noticed this effect in polarizers I scavanged from LCD screens and also the flexible plastic type polarizers they have in my physics lab.

Going by a simple wire grid model of a polarizer, this result baffels me completely. I wonder what causes it.

 

[mfb]

If you have played around with polarizers you might have noticed that flipping a polarizer by 180^o about the vertical axis has the same result as rotating it by 90^o about the horizontal axis[i.e. flipping so that the face of the polarizer which was facing away now faces you has the same effect as rotation by 90^o]

That depends on the type of polarizer and its orientation.

Some polarizers have an additional quarter wave plate included on one side (or even on both), to work as polarizers for circular polarization.

The light coming from my LCD monitor has a linear polarization.

 

[UltrafastPED]

I agree with mfb - you do not see this behavior in a simple polarizer such as a linear film or half wave plate.

 

[A.T.]

This might help:

https://www.youtube.com/watch?v=ycY2mUZHS84

 

[Drakkith]

This might help:

Nice link!
I didn't realize that was how circular polarizers worked.

 

[mfb]

That's exactly an application of "circular filters". The glasses have a quarter wave plate first and a polarizer afterwards (seen in the design direction for light).

- One side let's one circular polarization through, the other glass the other one. This way, you can tilt your head and still get the right images for both eyes. If they receive a linear polarization, they let 50% through.
- Towards the eyes, the emitted light is always polarized along one axis, where the axis is an arbitrary design choice.
- Light going in the wrong direction always leaves with a circular polarization.

The only way to block light are two polarizers behind each other - you have to flip one of the glasses.

 

[A.T.]

Nice link!

It does a good job of explaining circular polarization. But what I didn't find is a good animation or picture explaining what happens with the waves when multiple circularly polarizing filters are stacked. This would be interesting for these 6 cases (here with "reaL D 3D" glasses):

Note that:

- Most light comes through with same handedness and 180° flip (C), more than without the 180° flip (A).

- Blocking depends on the stacking order : See D vs. F and this video comparing the effect of 90° in F vs D:

https://www.youtube.com/watch?v=nVD9JA0hk_E

C vs. E is similar: Only C blocks when rotated by 90° while E is always clear.

 

[mfb]

Without a known polarizer, we cannot determine the full orientation of the elements in the glasses, but here is a possible arrangement:

left eye:
* quarter-wave plate "left-handed circular (I will call this LC) -> vertical (V)" (and right-handed circular (RC) -> horizontal (H)"
* vertical polarizer
=> LC goes through, RC is blocked

right eye:
* quarter-wave plate LC->H, RC->V
* horizontal polarizer
=> LC is blocked, RC goes through

Let's look at unpolarized light in setup A:
- 50% RC -> gets blocked at the first glass
- 50% LC -> goes through the first glass, afterwards it is V polarized. At the second glass, the quarter-wave plate converts V to RC (? should be checked), the V polarizer let's half of that through => in total 25% intensity

Let's look at unpolarized light in setup A:
Now the first element is a V polarizer, it let's 50% through (which is V polarized now). The following quarter-wave plate converts this to RC, and the following quarter-wave plate converts it back to V. The last element is the V polarizer, nothing happens. 50% of the light gets through.

All other setups can be analyzed in the same way, if you know how linear polarizers and quarter-wave plates work.

 

[A.T.]

All other setups can be analyzed in the same way, if you know how linear polarizers and quarter-wave plates work.

Yes, it helps to think in terms of these two components. For example the relative rotation of two stacked quarter wave plates: they either act as one half wave plate (swap polarization) or do nothing. Combined with the outer linear polarizers you get the effect in the video, that rotating the glasses has no effect.

I did a diagram on the 3D cinema. Is that correct, or one valid way to do it?:

 

[mfb]

Nice image, looks good!

 

[A.T.]

Nice image, looks good!

Thanks. Here is an idea I had, for a demonstration for kids . It works quite well. Obviously you need to wear a second pair of glasses to see the effect.