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:
http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/Graphics%5Cschem.BMP 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/RT006 randtk_TLC_Handbook.pdf

 

[Coolcrab]

I don't think that it has to do with reflection of the back, as they say that that's 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

 

[sardar]

I can provide an explanation for how thermochromatic liquid crystals change color. Liquid crystals are made up of molecules that have both liquid and crystalline properties. In the cholesteric phase, the molecules are arranged in layers that are slightly tilted with respect to each other. This arrangement allows for the reflection of light at a specific wavelength, known as the "pitch."

When the liquid crystals are heated, the molecules start to move and change their orientation, causing the pitch to shift. This shift in pitch changes the wavelength of light that is reflected, resulting in a change in color. This is similar to how a prism separates white light into different colors based on their wavelengths.

The specific color that is reflected depends on the temperature and the properties of the liquid crystal molecules. This is why different thermochromatic liquid crystals can change to different colors at different temperatures.

In terms of the image provided, it is important to note that liquid crystals are sensitive to polarized light. Therefore, the direction of the light entering the liquid crystals can affect the color that is reflected. This is why the diagram shows light entering from a specific direction to demonstrate the concept.

In summary, the change in color of thermochromatic liquid crystals is due to the change in the pitch of the molecules, which affects the wavelength of light that is reflected. I hope this explanation helps to clarify the phenomenon.