# Why birefringence effect polarizes light?

1. Aug 15, 2011

### Flavio

I am trying to understand birefringence effect. I have read many different sources online, but what I do not understand is why refracted light rays come out polarized (from a calcite crystal for example). What actually polarize light?
Can somebody help?

Thank you a lot!

2. Aug 15, 2011

### Pythagorean

the planes of the crystal lattice are made up the atomic components that make that lattice up. Here's an example of some lattices:

http://www.olympusmicro.com/primer/lightandcolor/birefringence.html

now imagine a line sweeping across these lattices. As it does so with the first lattice on the above page, it intersects a line of atoms: alternating red and green atoms in this cartoon depiction. Now imagine another line coming from the perpendicular direction. Again it sees red-green-red-green pattern. Now if you do the same experiment with the second lattice shape (b), you will find distinctly different patterns depending on the direction your line comes from. The third shape (c) is amorphous material, like a grey rock, not having much more than absorption for an optical property.

3. Aug 15, 2011

### Flavio

Thank you for the answer and link. However I still have some doubts.

From the website you pointed:
"Light entering an isotropic crystal is refracted at a constant angle and passes through the crystal at a single velocity without being polarized by interaction with the electronic components of the crystalline lattice."

- What does it mean that light is polarized by interaction with the electronic components of the crystalline lattice? How polarisation is triggered this way?

"[..] when light enters a non-equivalent axis, it is refracted into two rays each polarized with the vibration directions oriented at right angles to one another, and traveling at different velocities."

- Again, how comes the two rays are polarized?

"As we mentioned above, light that is doubly refracted through anisotropic crystals is polarized with the vibration directions of the polarized ordinary and extraordinary light waves being oriented perpendicular to each other."

The article is nice, but I still don't get where polarization originate.

Thanks again

4. Aug 15, 2011

### burochokkotti

What's your exact problem? Do you wanna know what's polarization or what's birefringence?

5. Aug 15, 2011

### Flavio

I want to understand why unpolarized light, after passing through calcite (for example), is polarized and why it is not unpolarized.
You may say because of birefringence effect, and that's ok. But I'd like to understand what's going on "in the clouds"...

Thanks

6. Aug 15, 2011

### Pythagorean

Ah. so light moves in a direction, but transverse to that direction, the filed amplitudes oscillate.

Now thy can oscillate at any angle transverse to propagation (so a whole plane of directions).

Natural light fromtje sun comes like this (every direction of polarization in a population of light particles). This is "unpolarized" light.

If all the light is produce coherently (like in a laser) it all lines up in the same direction (polarized).

But, you can also take unpolarized light and filter out waves that don't match your direction of interest with by passing it through a polarizing material (ie a lattice of atoms).

It only let's one direction of light through, so the light coming out is "polarized"

7. Aug 15, 2011

### Pythagorean

As for electrons, they absorb and emit photons, stepping up and down in energy state as they do so, sometimes letting the photon out at a different angle than it came in.

8. Aug 15, 2011

### Drakkith

Staff Emeritus
I believe it is because the electric field of the photons interact with the material differently depending on the orientation of the field. I'm not sure how specific and detailed you want to get. I don't know the actual geometry of the crystal lattice or anything.

9. Aug 16, 2011

### chrisbaird

The term "unpolarized light" may be the problem. Though clunkly, I think the phrase "randomly polarized with no average polarization" is more accurate. Think of unpolarized light as a vertically polarized beam plus a horizontally polarized beam, but they have a continually random phase relationship so they do not interfere. When this light hits the birefringent crystal, the horizontally polarized portion interacts with the crystal bonds and atoms along the horizontal direction, and the vertically polarized portion interacts with the crystal bonds and atoms along the vertical direction. If the crystal has the same atomic/bond structure in both directions, than the two parts of the unpolarized light are effected the same way, and the beam comes out the other side the same as it went in. If the crystal has an atomic arrangement in the horizontal direction that is different than in the vertical direction, then the two parts of the unpolarized beam will be effected differently, be bent different amounts due to refraction, and come out at different spots on the other side of the crystal. A birefringent crystal does not really create polarized light out of thin air, rather it filters the polarized components that are already present in unpolarized light.