I'm pretty sure we've all slipped on a mood ring and stared at it, waiting for it to change colors. It's exciting and fun, but how do they actually change colors?
Well, it's not super complicated, but it involves a bit of physics. In fact, years ago a physics textbook company used a photo of one of my mood rings in their textbook!
Here goes: the stone in a mood ring is a glass dome attached to a base with thermotropic liquid crystals in between. Liquid crystals are very sensitive to temperature (this is the same stuff that is in LCDs -- liquid crystal display, the screen you are reading this on!). In response to temperature changes, liquid crystal molecules change position, or twist. Hang on, this is where it gets technical!
This change in molecular structure affects the wavelengths of light that are absorbed or reflected by the liquid crystals. This results in a change in the color of the stone. For example, as the temperature increases, the liquid crystal molecules twist slightly in one direction. This twist causes the liquid crystal substance to absorb more of the red and green portions of the visible light, and reflect the blue part. This causes the stone to appear dark blue. When the temperature decreases, the molecules begin to twist in the other direction, and reflect a different portion of the spectrum.
So this is the fun part! The metal of the ring conducts heat from your finger to the liquid crystals in the mood stone and voila! Color change!
Now the next question is, Do Mood Rings Really Work?