Is a polarizer a filter or a converter?

[Joncon]

There is absolutely no evidence this is a true assessment. After any photon passes through a 0 degree polarizer, it will continue to pass similarly oriented polarizers - no problem. However, its orientation at 45 degrees is completely indeterminate. So I would NOT say it is oriented at 45 degrees. For if it were, it would pass 45 degree polarizers.

Yes, that's what I was thinking.

Is it correct then to say that, while a polarizer acts like a filter (either letting photons pass or not), it also "rotates" the polarization of photons which pass successfully?

 

[DrChinese]

Is it correct then to say that, while a polarizer acts like a filter (either letting photons pass or not), it also "rotates" the polarization of photons which pass successfully?

It does appear to have that effect. Any quantum observation has this appearance (following the HUP). It is a bit difficult to know exactly what is happening during the process of collapse, and there are varying interpretations.

 

[ibysaiyan]

It does appear to have that effect. Any quantum observation has this appearance (following the HUP). It is a bit difficult to know exactly what is happening during the process of collapse, and there are varying interpretations.

Are there any links/articles which go a little in depth about what happens during the collapse of the wavefunction ? I will be grateful if you know of any.

P.S: Sorry for having hijacked this thread, slightly.


-ibysaiyan

 

[harrylin]

I have yet to see where a polarizer modifies the polarization of individual photons.

The point of the demonstration with three polarizers - as also Joncon and others tried to explain - is that evidently photons that come out of the middle polarizer have a different polarization than the photons that enter it. If the photons coming out had the same polarization as the ones that enter it, then no light would come out. It's a basic characteristic of polarizers that they attenuate by changing the polarization, in contrast with pure attenuators that do not change the polarization.

 

[Drakkith]

There is absolutely no evidence this is a true assessment. After any photon passes through a 0 degree polarizer, it will continue to pass similarly oriented polarizers - no problem. However, its orientation at 45 degrees is completely indeterminate. So I would NOT say it is oriented at 45 degrees. For if it were, it would pass 45 degree polarizers.

Ah, ok. I had not seen anything like that before.

I found this page that explains the effect: http://demonstrations.wolfram.com/LightBeamsThroughMultiplePolarizers/

The point of the demonstration with three polarizers - as also Joncon and others tried to explain - is that evidently photons that come out of the middle polarizer have a different polarization than the photons that enter it. If the photons coming out had the same polarization as the ones that enter it, then no light would come out. It's a basic characteristic of polarizers that they attenuate by changing the polarization, in contrast with pure attenuators that do not change the polarization.

Yes, I see now. It would appear that a polarizer is both a filter and a converter then.

 

[sgb27]

We have a bowl filled with objects in different colors and different shapes. If we filter out all red objects, there is no way to get any red objects back by filtering out other objects (cubes for example). This is the behaviour I expect from filters, but it's not the way polarizers behave. If I filter out one part of the whole wave (for example the 0°-polarized), I can get it (partially) back by filtering out another part (the 45°-polarized).

Consider this. The 1st polariser filters out red balls, the 2nd yellow balls, and the 3rd green balls. Now consider that red balls are equal to (yellow-green) balls, and green balls are equal to (yellow-red) balls. You'll see that the 2nd filter will "turn" green balls into (negative) red balls, but it's not adding anything, only subtracting the yellow component.

Maybe the bit that is confusing is that a light wave polarised in the negative "red" direction is just as visible as one polarised in the positive "red" direction. With real objects you can't see a "negative red" ball.

 

[harrylin]

Consider this. The 1st polariser filters out red balls, the 2nd yellow balls, and the 3rd green balls. Now consider that red balls are equal to (yellow-green) balls, and green balls are equal to (yellow-red) balls. You'll see that the 2nd filter will "turn" green balls into (negative) red balls, but it's not adding anything, only subtracting the yellow component. [..]

Your illustration doesn't work. For it to work, no ball at all must pass with two filters, and some balls must pass with an additional filter added between them. That won't be easy.

Harald

 

[sgb27]

Your illustration doesn't work. For it to work, no ball at all must pass with two filters, and some balls must pass with an additional filter added between them. That won't be easy.

Harald

Assuming you just start with red and green balls, the red will be filtered out by the 1st polariser and leave the green unchanged, and the 3rd polariser (at 90 degrees) will filter out the green leaving no balls passing. The key point is that each polariser considers the "colours" relative to it's optical axis, the 1st and 3rd use red/green as their axes, and the 2nd uses another set of colours. I think that's about as good as you can get with this analogy.

 

[harrylin]

Assuming you just start with red and green balls, the red will be filtered out by the 1st polariser and leave the green unchanged, and the 3rd polariser (at 90 degrees) will filter out the green leaving no balls passing. The key point is that each polariser considers the "colours" relative to it's optical axis, the 1st and 3rd use red/green as their axes, and the 2nd uses another set of colours. I think that's about as good as you can get with this analogy.

Indeed the analogy only serves to show that a mere filtering doesn't work: either a filter filters out a full colour (which isn't the case in your example, the second filter distinguishes and filters a component), or it only filters out a component (which isn't the case in your example, the third filter doesn't let the yellow component pass).

 

[sgb27]

In terms of polarisers there is no such thing as a "full colour", ie an absolute or reference axis for polarisation. You must consider every case with reference to the optical axis of the material being considered.

Even in the analogy, who said that red was a "full colour" and yellow wasn't? Red can be made up of yellow and negative green, and also yellow can be made up of red and positive green. It all depends on the point of view of the specific filter/polariser as to what is the "full" colour. In terms of the 2nd polariser, yellow is the full colour, it no more or less full than the red the first polariser sees.