# Polarized light - QM vs. classical?

1. Apr 20, 2005

### DaveC426913

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: Nov 28, 2006
2. Apr 20, 2005

### shyboy

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

3. Apr 20, 2005

### Gonzolo

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.

4. Apr 20, 2005

### DaveC426913

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.

5. Apr 20, 2005

### Claude Bile

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 infinte number?).

Claude.

6. Apr 20, 2005

### DaveC426913

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?

7. Apr 21, 2005

### Staff: Mentor

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.

8. Apr 22, 2005

### DaveC426913

OK cool.

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