Linear Polarization: Active or Passive Element?

[ProTerran]

Hi, is a linear polarizer acting like an active element that rotates polarization angle or is it just a passive element?

I wonder about this because there is this experiment with three polarizers, where two of them are rotated 90 degree to each other so no light can pass through, and then third polarizer is put between those two with i.e. 45 degrees so that light can pass.

What physical phenomena is behind this?

Best regards.

(Note: English isn't my mother language)

 

[Antiphon]

It is a passive device. It is a classical experiment as well and does not require quantum mechanics to explain how it works.

The polarizer can be made of a fine grid of wires in the vertical direction. When light strikes it, it makes current flow up and down the wires. If the incoming wave is vertically polarized it will be reflected by the wires. If horizontally polarized it will not. At 45 degrees, half the light is reflected and half transmitted *and rotated* to vertical linearly polarized light by the current flow and orientation of the wires.

Edit: you can view this as a decomposition into two waves one of which is passed by the polarizer and one which is not. In this case the polarizer does not rotate the wave but just filters it. Both are mathematically and physically equivalent.

 

[ProTerran]

Thanks for your response.

So does that mean that the current flow induced in "wires" by incident light on the polarizer surface is causing rearranging of the light polarization and absorption/reflection of the light whose polarization angle is lined up with the polarizer "wires"?

 

[Antiphon]

Yes, exactly.

 

[sardar]

I can provide an explanation for the question about linear polarization and whether it is an active or passive element.

First, let's define what we mean by active and passive elements. An active element is one that actively changes or manipulates the properties of light, while a passive element is one that simply allows light to pass through without altering its properties.

In the case of linear polarizers, they can be considered passive elements because they do not actively change the polarization angle of light. Instead, they have a fixed orientation that only allows light with a specific polarization angle to pass through.

Now, let's discuss the experiment with three polarizers. When two polarizers are placed perpendicular to each other, no light can pass through because the first polarizer blocks all light with a certain polarization angle, and the second polarizer blocks all light with the perpendicular polarization angle. However, when a third polarizer is placed between them at a 45-degree angle, some light is able to pass through.

The physical phenomena behind this is called the Malus' law, which states that the intensity of light passing through a polarizer is proportional to the cosine squared of the angle between the polarizer and the initial polarization angle of the light. In this case, the third polarizer is at a 45-degree angle to the initial polarization angle, so the cosine squared of 45 degrees is 0.5. This means that half of the light is able to pass through the second polarizer.

I hope this explanation helps clarify the role of linear polarizers as passive elements and the physical phenomena behind the experiment with three polarizers. If you have any further questions, please do not hesitate to ask. Best regards.