# Variable Index of Refraction!

1. Oct 11, 2005

### Erkenbrand

Hi guys.
I'm doing this science fair project and I needed to know if there is a substance (any substance!) whose index of refraction can be changed/regulated by applying an electric current (or any other action) to it.

I appreciate all help. Thanks!

2. Oct 11, 2005

### Danger

It changes in water by boiling or freezing it.

3. Oct 11, 2005

### Danger

I still can't edit, so here's a PS. I don't know whether or not the index changes in an LCD, but the transparency certainly does.

4. Oct 11, 2005

### ZapperZ

Staff Emeritus
Liquid crystal, such as your LCD displays.

Zz.

Edit: Argh... Danger beat me to it.

5. Oct 11, 2005

### ZapperZ

Staff Emeritus
Try clearning your cache, Danger. I find doing that seems to solve a few of the problems since the upgrade.

Zz.

6. Oct 11, 2005

### Danger

How on Earth does one do that? Perhaps it's the PC equivalent of rebuilding my desktop? Zapping my PRAM?
Anyhow, I just realized that since I'm at work now, I can edit. Just can't do it at home.

7. Oct 11, 2005

### Erkenbrand

Thanks guys. Water doesn't really work for me. Now, I'm interested in LCDs. I understand they have the property of birefringence, and electric currents change the alignment of the crystal. Now, is this binary (two possible states) or can you get the crystals to arrange in a gamut of ways?
Thanks again.

8. Oct 11, 2005

### Danger

Although I don't know the exact response mechanism, you can get gradual transition from clear to fully opaque depending upon the input. My glasses do the same thing, but are triggered by light rather than electricity. Whether it's due to the crystals only partially rotating, or to only some of the crystals fully rotating, is unclear to me.

9. Oct 11, 2005

### Erkenbrand

Hmmm. Apparently what happens when you pass a current through an LCD, the crystals "untwist" and align at 90° angles. Therefore it is binary (no partial rotation). Am I correct?
If so, then is there anything that can function similarly to an LCD, but rotate at different angles depending on the intensity of the current? If not, I return to my original question. Is there a substance whose index of refraction can be changed with a current?
Thanks guys.

10. Oct 11, 2005

### Claude Bile

Electro-optic materials are materials whose refractive index varies with voltage (more accurately, the applied electric field) rather than current (although the voltage required is typically in kV, which is a safety issue).

Any crytsalline material that lacks a centre of symmetry will exhibit this behaviour. Some commonly used crystals are Lithium Niobate, Quartz, Gallium Arsenide, Zinc Selenide and Lithium Tantalate.

If you do choose to play with electro-optic materials, be aware of the dangers associated with the high voltages that are often required to work these devices.

Claude.

11. Oct 12, 2005

### Erkenbrand

Thanks, Claude. Yeah, I was actually thinking of lesser voltages (not even 10V!!). But I'll research electro-optic and thermo-optic materials.

12. Oct 12, 2005

### mezarashi

I was just studying electro-optic materials today used in laser modulation. It would seem like Lithium Niobate would be a good bet $$LiNbO_3$$. Semiconductor materials are known to have variable indices of refraction as well, except they don't work as well and they are generally opaque to visible light unless they are properly junctioned and biased.

13. Oct 12, 2005

### Erkenbrand

Awesome. I'll look it up. Is it... hard to find -- Lithium Niobate, I mean (the substance)?

14. Oct 12, 2005

### mezarashi

I would suppose it's a bit difficult to find. The material is relatively common in electronics even in TVs. For example some commercially available laser modulators use this. They are not very cheap however.

Especially since you want to demonstrate this at a science fair. Maybe it is better you get a spare laser modulator (or maybe found in a junk shop) configured for amplitude modulation. Applying a voltage, you can show that the light goes on and off accordingly.

Last edited: Oct 12, 2005
15. Oct 12, 2005

### Claude Bile

Lithium Niobate is widely available, but mostly from companies that specialise in optical materials. A wafer of Lithium Niobate is relatively expensive however. Quartz is probably a cheaper option.

Claude.