system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do Transition glasses lenses work?

I always see the adds on how they change in tint automatically depending on the brightness, but how do they do this?
gpt
Ultraviolet light breaks a chemical bond in a special dye that coats the lens surface. Before UV exposure, the molecules in this dye are colourless. UV breaks a certain bond in the molecule, changing the structure, and resulting in a product that is coloured. When you go back indoors, away from UV, the reaction goes the other way, and the lens becomes clear again. Some things to consider if you are thinking about Transitions lenses: 

* They won't change colour in a car, since your windshield blocks most of the UV light entering the vehicle. There are certain types of Transition lenses which do change somewhat behind a windshield. These usually have side effects such as a remaining tint indoors, or also responding to visible light.

* They are affected by temperature. Specifically, they do not get as dark in hot temperatures as they do in cold temperatures, and they will go back to being clear faster in hot temperatures than in cold. Heat is what drives the reaction back towards the clear side. If you are an avid beach goer, separate prescription sunglasses may work better for you (but can be very expensive).

* A type of Transitions lenses are available where the dark form of the lens is polarized, which cuts glare better. These do tend to have a slight tint indoors, but can be a nice lens when in the dark state.

* The expected lifespan of the colour changing properties of these lenses is 3-4 years.

* Adding a colour changing feature to your glasses will up the cost. However, it may increase the utility that you get out of them.

* Transitions is actually only one brand of photochromic (colour changing) technology for eye glasses. There are several others, but Transitions is the most widespread and best marketed.

* As with any eye glasses, leaving them in your car (especially on your dashboard) where they will experience wide temperature swings is bad for them, and breaks down the coatings. Don't do this.