Thermochromatic liquid crystals, how do they change color?

Coolcrab

If you look at liquid crystals in things like moodrings and LCthermometers then you will notice that they can change color when heated.

Now I have looked on the internet and found that it has to do with the The cholesteric (or chiral nematic) liquid crystal phase, where they align with each other in layers but are slightly tilted across layers. Example:


I also know that they reflect the light which has the wavelength of a full rotation of the crystals, this being called the "pitch". (see pic)

So if they rotate at such an angle that they make a full rotation in P = lambda Red, then it will look red.

But I do not know why. If I look at this picture, then it looks like a polariser that goes in all directions:
Which would cansel out all the light, and not reflect any.

So can anyone tell me what's going on here?

Andy Resnick

Don't forget, light passes through the LC dye twice- there's a reflection off the back, and reflection also adds a phase change to the light.

I found this:

http://www.hallcrest.com/downloads/R...C_Handbook.pdf

Coolcrab

I don't think that it has to do with reflection of the back, as they say that thats often black to absorb everything that comes trough.
So it has to reflect all red light, while letting trough all the other light. But I don't see why or how.

And yea I, saw that source :P

Coolcrab

I now know that the poleriser would not cansel out all the light, but rather repolerise/absorb a bit.
But I still don't get why red light would be reflected, while the rest is let trough.

Edit: red light in the case the the pitch has the length of red light's wavelength