# Flipping a polarizer 180deg. changes polarization

• i_amitabh
In summary, a polarizer with a quarter wave plate on one side lets more circular polarization through than a linear polarizer with no quarter wave plate. This can be seen in the design direction for light on an LCD screen.

#### i_amitabh

If you have played around with polarizers you might have noticed that flipping a polarizer by 180o about the vertical axis has the same result as rotating it by 90o 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 90o]

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.

i_amitabh said:
If you have played around with polarizers you might have noticed that flipping a polarizer by 180o about the vertical axis has the same result as rotating it by 90o 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 90o]
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.

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

This might help:

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

1 person
A.T. said:
This might help:

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

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.

Drakkith said:
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.

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.

mfb said:
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?:

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• 3d_glasses_left.png
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• 3d_glasses_right.png
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Nice image, looks good!

mfb said:
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.

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• shadow_play.jpg
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## What is the concept of flipping a polarizer 180 degrees?

Flipping a polarizer 180 degrees refers to rotating a polarizing filter by half a circle, causing the direction of the polarization to reverse.

## Why does flipping a polarizer 180 degrees change the polarization?

Polarizing filters are designed to only allow light waves that are oscillating in a specific direction to pass through. Flipping the filter 180 degrees changes the direction of the filter, therefore allowing a different set of light waves to pass through.

## What is the scientific explanation behind the change in polarization when flipping a polarizer 180 degrees?

Flipping a polarizer 180 degrees changes the orientation of the molecules within the filter. These molecules are responsible for filtering out certain light waves, so when their orientation is reversed, the filter allows a different set of light waves to pass through.

## How does flipping a polarizer 180 degrees affect the intensity of light passing through?

The intensity of light passing through a polarizer is dependent on the angle of the polarizer in relation to the incoming light waves. Flipping the polarizer 180 degrees does not affect the intensity of the light, as it is still passing through the same filter at a different angle.

## Can flipping a polarizer 180 degrees change the polarization of all types of light?

Yes, flipping a polarizer 180 degrees can change the polarization of all types of light, including visible light, infrared light, and ultraviolet light.