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02-07-2014, 01:10 PM   #1
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polarising filters question?

Hi all,


I have studied several posts on polarizing filters, especially circular polarisers. I think I understand the explanation. However -


* Upon looking through the filter to a mirror and through the same filter again, it is darkened a little when the thread is pointing towards you.
* Upon looking through the filter to a mirror and through the same filter again, it is fully blacked out when the thread is away from you.


How come?


I thought the lens side of a CPL fully rotates the light to all directions again. A regular mirror, as far as I understand, does not rotate any light. Therefore, if anything, the light beams between the mirror and the lens, should be the same orientation and pass that same filter without a problem. This does not happen. Argumentum ad Balucum. So what is going on?


Puzzled by this result I pulled out my 3D movie glasses. They use polarisation to have the left and right eye see a different part of the screen.
* The same experiment as above is repeated with both glasses. The effect is exactly reverse as above. When the eye-faceing part is facing you it is blacked out. When the glasses are reversed they transmit light.


Anyone has a clarification?

02-07-2014, 01:21 PM   #2
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QuoteOriginally posted by JohntheThird Quote
Hi all,


I have studied several posts on polarizing filters, especially circular polarisers. I think I understand the explanation. However -


* Upon looking through the filter to a mirror and through the same filter again, it is darkened a little when the thread is pointing towards you.
* Upon looking through the filter to a mirror and through the same filter again, it is fully blacked out when the thread is away from you.


How come?


I thought the lens side of a CPL fully rotates the light to all directions again. A regular mirror, as far as I understand, does not rotate any light. Therefore, if anything, the light beams between the mirror and the lens, should be the same orientation and pass that same filter without a problem. This does not happen. Argumentum ad Balucum. So what is going on?


Puzzled by this result I pulled out my 3D movie glasses. They use polarisation to have the left and right eye see a different part of the screen.
* The same experiment as above is repeated with both glasses. The effect is exactly reverse as above. When the eye-faceing part is facing you it is blacked out. When the glasses are reversed they transmit light.


Anyone has a clarification?
See this explanation: http://www.lhup.edu/~dsimanek/14/polaroid-short.htm
02-07-2014, 01:45 PM   #3
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Thanks Rich for the clarification.


So in the mirror, it is the retarding layer / recirculating layer that blocks out the light, not the polarising layer?
02-07-2014, 05:10 PM   #4
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Hi

I thought it might be an appropriate place to ask an additional question.

Why are all polarisers coloured, i.e. grey.
I have an old ROBOT film camera and for it I have a linear polariser which is clear. Well almost, when I put it on a white sheet of paper I can see a very week brownish colour. By just looking through it this weak colour is not apparent, so I call it really clear.

Any of you learned folk here know the answer ?

Greetings

02-07-2014, 05:51 PM   #5
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gray is not a color, it is white on the way to black! Two white sheets next to each other--one with less light on it looks gray compared to the brighter (white) one.

BTW in engineering a gray surface is somewhat (slightly) different. It is one that reflects less than all of the light falling on it--but it does so equally for all wavelengths.
02-07-2014, 07:39 PM   #6
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QuoteOriginally posted by dms Quote
gray is not a color, it is white on the way to black! Two white sheets next to each other--one with less light on it looks gray compared to the brighter (white) one.

BTW in engineering a gray surface is somewhat (slightly) different. It is one that reflects less than all of the light falling on it--but it does so equally for all wavelengths.
Thanks for the lecture. I know all that but it does not answer my question.
The word colour and grey was just used as a way to describe what you explain as: grey is not a colour, it is...
I am sorry you felt compelled to set the record straight; Not in my case but maybe someone else will benefit from it.

...one with less light on it looks gray compared to the brighter (white) one. I is called Metamerism if the colour, intensity or wavelength of the source light changes.

The question still remains, why has a polariser got to have this "non colour white on the way to black" appearance.
Note, the word colour was not used.

Greetings

Last edited by Schraubstock; 02-07-2014 at 07:49 PM.
02-07-2014, 08:04 PM   #7
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Was not meant to be a lecture: you asked why a polarizer is colored i.e., gray?

I have no idea what else you meant--as if it was passing blue light it would be darker blue, not gray.

---------- Post added 02-07-14 at 08:11 PM ----------

Sorry to repeat--but the PL does not pass all the light so it is darker, and the darker "white" looks gray. Some PL lose 1.3 e.v., and some I believe lose closer to 2 e.v., so the latter would be a darker gray. And if you combine 2 PL to act as an adjustable ND filter it can go to essentially black (although at that point it is said it will pass a blueish color--anyway a linear one does).

Last edited by dms; 02-07-2014 at 08:17 PM.
02-07-2014, 08:32 PM   #8
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QuoteOriginally posted by dms Quote
Was not meant to be a lecture: you asked why a polarizer is colored i.e., gray?

I have no idea what else you meant--as if it was passing blue light it would be darker blue, not gray.

---------- Post added 02-07-14 at 08:11 PM ----------

Sorry to repeat--but the PL does not pass all the light so it is darker, and the darker "white" looks gray. Some PL lose 1.3 e.v., and some I believe lose closer to 2 e.v., so the latter would be a darker gray. And if you combine 2 PL to act as an adjustable ND filter it can go to essentially black (although at that point it is said it will pass a blueish color--anyway a linear one does).
Again I know all that It still does not explain why my old lin. polariser remains clear under any light.

Greetings

02-08-2014, 01:28 AM   #9
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A fair question by Schraubstock: Why is a linear polariser more clear than a circular polariser? (at least if I do understand your question...)


Could be that retarder screen which is behind the circular one. You could try a little experiment with both filters and a mirror, see what happens.

---------- Post added 02-08-14 at 09:43 AM ----------

Rich,


Your link also states that the 3D cinema glasses can be used in reverse as a filter for your lens. That is very cool to know. Supposedly, they work "just as well".


Then why the glasses cost 1 Euro and the filters 80 Euro?
02-08-2014, 04:48 AM   #10
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QuoteOriginally posted by dms Quote
gray is not a color, it is white on the way to black!
Officer to witness: Did you see the bank robbers get away ?

Witness: Yes Officer, Sir, by car.

Officer: What colour was the car.

Witness: Officer, the car had no colour.

Officer: What, you mean the car was colourless ?

Witness: Yes Sir, it was white on the way to black !



Very sorry I couldn't resist
02-08-2014, 11:30 AM   #11
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Your scenario reminds me when someones stopped their car and:

asked me if I knew the way to a particular store,

and I said yes!
02-10-2014, 09:01 PM   #12
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QuoteOriginally posted by JohntheThird Quote
Thanks Rich for the clarification.


So in the mirror, it is the retarding layer / recirculating layer that blocks out the light, not the polarising layer?
Light goes through the polarizing part and becomes linearly polarized then through the quarter-wave plate to make it circularly polarized. That light hits the mirror and reflects back (the relevant reflected beams are the ones that reflect at an angle of reflection small enough that they will pass through the plane of the polarizer again). The mirror does not affect polarization, but does affect direction of propagation. Let's trace the rays and see how this leads to complete destructive interference, and hence, complete blackness.

You stand behind the polarizer. (Unpolarized) Light enters the filter and first becomes linearly polarized, say at 90 to the horizontal. Then it goes through the quarter wave plane and becomes circularly polarized; based on your perspective, it will be rotating clockwise. Then it hits the mirror and comes back; this doesn't change the polarization, but does change the direction of propagation. From your perspective, now the light will be rotating counterclockwise when it goes back through the quarter-wave plate. Coming through the other side, the plate will now transform in the opposite manner.

In (forward direction, from your perspective): 90 linear --> clockwise circular or -90 --> counterclockwise circular
Out (from your perspective): clockwise circular --> -90 linear, counterclockwise circular --> 90 linear

So our light exits the plate at 90 linear polarization. Now it heads through a linear polarizer that took unpolarized light and made it linearly polarized 90 light. So...reversing it means you get...-90 degree light. Between the quarter plate and the polarizer you have light at nearly identical amplitudes* with polarizations of the same direction that differ by 180. Hence, they cancel in total destructive interference** and you see nothing.


*: The light reflected is not identical to the light that comes through, even if the scene is static. Even light can't be in two places at once (finite speed of propagation). But the two are so nearly identical due to the ridiculous speed of light compared to the distance that the light very nearly IS in two places at the same time. At the least, your eyes can't tell that it's different.

**: Of course, the filters aren't perfect and you won't have quite 180 phase mismatch, but it will be close. That's why you can see some light, but not a lot. The better the filter, the less you should see.
02-10-2014, 09:37 PM   #13
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QuoteOriginally posted by Schraubstock Quote
Again I know all that It still does not explain why my old lin. polariser remains clear under any light.
I'm guessing, from your brief description that the linear polarizer is a single sheet, whereas the CPL is a double layer. Any single polarizing sheet should be fairly clear. But two sheets, even when aligned the same way will start to get grey because the alignment isn't perfect. A CPL also has a layer for the quarter wave plate plus the second polarizing layers, and so the combination is what is darkening the light coming through, making it grey.
02-11-2014, 02:48 AM   #14
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QuoteOriginally posted by MSL Quote
I'm guessing, from your brief description that the linear polarizer is a single sheet
Hi

You could well be right but I have no way of telling short of a destructive test.
Thanks for your post
02-13-2014, 03:15 AM   #15
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Well that means your all a lot cleverer than me on this thread, you've all left me behind somewhere on the last curve.
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