Have you ever seen a t-shirt that changes colour when it is in direct sunlight? Or perhaps new colours become visible? Maybe you have some of those fancy prescription glasses that become tinted sunglasses as you step outside.
All that falls under the name Photochromism. Photo being light, and chromo being colour.
It’s a minutely complex chemical reaction that occurs in the presence of electromagnetic waves in the ultraviolet region of the spectrum. That is to say, UV light. When those UV waves hit the photochromatic substance, the chemical reaction occurs that causes a change in its ability to absorb visible light.
Now, here at LSNED I always try to take the hardcore science and boil it down to a casual explanation that makes sense. Just so you’re aware of my struggle, here’s the source material on this…
The conjugated system of the oxazine and another aromatic part of the molecule is separated by a sp³-hybridized “spiro” carbon. After irradiation with UV light, the bond between the spiro-carbon and the oxazine breaks, the ring opens, the spiro carbon achieves sp² hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals with the rest of the molecule, and a conjugated system forms with ability to absorb photons of visible light, and therefore appear colorful.
So, umm, to summarize, err… well… so every thing in the universe is made up of the same building blocks, as seen on the Periodic Table of the Elements. The difference between your eyeball and the computer screen you’re looking at is just a different arrangement of those basic parts. In the case of our colour-changing dye, the sunlight actually rearranges the physical bonds between the parts, and that teeny-tiny change makes for a dramatic and visible shift in the objects properties. Just like your eyeball, if the parts suddenly got scrambled, could end up being a potato dumpling.
When it comes to glasses that turn into sunglasses as you step outside, that chemical process is a little bit different. When they make the lenses, they add two special ingredients. Silver chloride and copper chloride. When that same UV light hits the silver chloride it oxidizes, or “rusts”. That throws the physical arrangement out of whack and leaves us with a pile of leftover electrons, which find new homes with silver ions to create new silver atoms that block some of the incoming light, making the glasses seem darker.
It might stay that way for a ling time, if not for the copper chloride that was also added into the lens. Like an atomic groundskeeper, the copper chloride not only keeps the stray chlorine from escaping the lens, but it reduces and re-sets the chemical change as soon as the UV light has faded away. The tinting process is reversed, and set to go again.
Personally, I find chemistry to be the most mysterious thing in the universe. I tend to have a logical grasp on the concepts of physics, but chemistry still seems like magic to me!