If you’re thinking about getting transition lenses, consider this: Every time you do something as simple as walking out of a building, you could watch a chemical reaction happen literally right in front of your eyes. 

Armed with chemical compounds that spring to action under ultraviolet light, transition lenses darken even on cloudy days to keep out those damaging rays. Then, when the coast is clear, they simply return to transparency. 

Transition or “photochromic” glass was originally developed in the 1960s by Donald Stookey, a chemist at Corning Glass Works and a prolific inventor. (Stookey is most famous for discovering the super durable and extremely popular kitchenware material known as CorningWare, which he actually found accidentally after setting up a test reaction at 900°C instead of 600°C.) Soon after Stookey patented the material, Roger Araujo, another Corning chemist, used his breakthrough to develop the first photochromic lenses.

In 1965, Corning commercialized the first generation of transition lenses under the brand “Bestlite.” Three years later, these were dropped in favor of the more reliable Photogray lenses, named for their bluish gray hue when darkened. This color comes from the tiny amounts of the compound silver chloride (<0.1 percent) dispersed throughout the glass. When exposed to UVA light (315 nm – 400 nm), silver gains an electron from chloride to become silver metal, and gets the ability to absorb visible light and appear darker. They found that this reaction would work with any halogen or element from the same column in the periodic table as chlorine that is capable of giving away one electron to silver. 

The same darkening process is also used for developing photographic film except that film exposure is permanent, while photochromic lenses possess another component, such as copper chloride, that helps silver return to its original, non-absorbing state once it’s away from UV light. 

With the introduction of plastic lenses in the 1980s came the next generation of transition lenses based on thin films of organic compounds. These mostly carbon molecules—such as pyridobenzoxazines, naphthopyrans, and indenonaphthopyrans—react to UVA light by rearranging their chemical bonds into new species that can absorb and essentially block UV and visible light. Like tiny transformers, they can switch between either form depending on the presence or absence of UV light.

Plastic transition lenses are lighter and thinner than their glass counterparts, but their organic films are more susceptible to degradation than the silver halides used in glass.

But for both glass and plastic transition lenses, the darkening process happens almost instantaneously, while becoming clear takes anywhere from three to five minutes—which can be disorienting indoors. The clearing reaction is so much slower because it can’t rely on the driving energy of UV light. One trick to speeding up the reaction is to add heat energy by running the lenses underneath warm water.

Another inconvenience that can’t be avoided as easily comes from modern car windshields. Some are specially designed to filter out UV light, making it difficult for lenses to activate the darkening effect needed for driving.

Transition glasses may or may not be right for you, but they’re an excellent example of the everyday chemistry that’s happening in plain sight.