# Polarized light - QM vs. classical?

• DaveC426913
In summary, this phenomenon is a demonstration of classical physics, in which light passing through a polarizing filter changes its polarization angle.
DaveC426913
Gold Member
Could someone 'splain this phenomenon?

I place two polarized filters (A and B) at right angles to each other so that the first filter passes light that's polarized vertically, while the second passes light that's polarized horizontally. No light gets all the way through. OK so far.

I now place a third polarized filter (C) between the first two. This one I angle so that it's diagonal (45 degrees from both A and B). Suddenly, some light gets through!

Obviously, the act of polarizing the light as it passes through the first filter is having an effect on its subsequent polarization.

Can someone explain what is happening?

I read somewhere that this is a clear demonstration of QM. Is there a classical explanation?

Last edited:
let me ask you one question- what is the light polarization after filter C?

The direction of linear polarization (after A) can be defined by a single vector. This vector can be seen as the sum of two perpendicular vectors, of which only one goes through C. This remaining vector can then also be seen as a sum of two perpendicular vectors, only one of which goes through B.

Sorry, I was a little ambiguous in my diagram.

For purposes of discussion, light will pass through B *then* through A (left to right).
So that when C is added, light goes B > C > A. (In reality, it doesn't matter. It will work both ways BCA or ACB, but let's keep our numbers straight.)

So, what is the polarization after C? (i.e. en route from C to A).

Actually, I don't know. Well, I guess what's remaining will be diagonally polarized. So when it gets to A, some of it wil get through.

But perhaps I'm also not being clear about something else. There is MORE light coming through after passing through A+B+C than there was after passing through any two filters (A+B, A+C or B+C). How is that possible?

"The direction of linear polarization (after A) can be defined by a single vector. This vector can be seen as the sum of two perpendicular vectors, of which only one goes through C. This remaining vector can then also be seen as a sum of two perpendicular vectors, only one of which goes through B."

I see how that works (oddly enough), so I guess the problem is I don't understand how polarization actually manifests in light.

This confusion arises when polaroids are though of filters, because of the inevitable paradox that arises, i.e. if I add a filter, I get more light through. Polaroids are actually a little more complicated than that as they rotate the polarisation vector of the incident field.

As an interesting thought experiment, consider the situation where there are many polaroids in between A and B, (possibly an infinite number?).

Claude.

Claude Bile said:
As an interesting thought experiment, consider the situation where there are many polaroids in between A and B, (possibly an infinite number?).Claude.

Yes well, I'll leave that until afrter I understand the basic model with just 3, OK?

OK, so light passing through a polarizing filter does change the nature of the light?

This is not a QM effect then? It is merely classical physics?

If you think of the light as an electromagnetic wave, the orientation of its E field represents its polarization angle. The nature of an ordinary dichroic polarizer is to strongly absorb the E field in one direction, allowing the field perpendicular to that direction to pass through. The light that makes it through the polarizing "filter" has its field realigned with the polarization of the filter. Ignoring loss, if polarized light ($E_0$) passes through a dichroic filter at an angle $\theta$, the transmitted beam is now polarized parallel to the orientation of the filter's axis and has a field strength of $E_0 \cos\theta$. The intensity of the beam is reduced to $I_0\cos^2\theta$--the law of Malus.

As Claude points out, it does seem kind of weird that adding a filter could increase the amount of light transmitted; but if you look at it step by step, it makes sense. Each filter reduces the intensity of the light passing through it, but that change in polarization angle makes all the difference.

OK cool.

And drat. I thought I was looking at a direct application of QM.

## 1. What is polarized light?

Polarized light refers to a type of electromagnetic radiation that has its electric field oscillating in a specific direction. This means that the light waves are aligned and vibrate in the same plane, unlike unpolarized light which has randomly oriented electric fields.

## 2. What is the difference between classical and quantum mechanics in terms of polarized light?

In classical mechanics, polarized light is described as a wave phenomenon that can be fully explained by Maxwell's equations. However, in quantum mechanics, polarized light is described as a stream of photons, each with a specific polarization state, and its behavior is governed by the laws of quantum mechanics.

## 3. Can polarized light be observed in everyday life?

Yes, polarized light can be observed in everyday life through various phenomena such as glare reduction on sunglasses, 3D movie glasses, and LCD screens. Natural occurrences such as the blue color of the sky and the rainbow are also examples of polarized light.

## 4. How is polarized light used in scientific research?

Polarized light is commonly used in various scientific research fields, including materials science, chemistry, and biology. It is used to study the structure and properties of materials, identify certain molecules, and understand the behavior of light in different environments.

## 5. Can polarized light travel through all types of materials?

No, polarized light can only travel through materials that allow for the transmission of electric fields in a specific direction. Materials such as glass, plastic, and certain crystals can transmit polarized light, while others like metals and opaque materials cannot.

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