Japanese researchers have made glass that darkens under UV light, but doesn't contain the harmful chemicals now found in sunglasses and other light-sensitive glass.
Scientists at the National Institute of Advanced Industrial Science and Technology in Osaka developed the glass, but don't expect to be able to buy shades made from it for a while.
The chemicals that cause some sunglasses to darken in the sun are the same ones used in photographic film: compounds of silver and a halogen, such as chlorine or iodine.
The compound normally exists as positively charged silver ions and negatively charged halogen ions. When the compound is exposed to light, electrons from the halogen ions can jump to the silver ions, converting them to silver metal.
Atoms of silver clump together and scatter light, turning the glass dark. In sunglasses, the glass goes clear again when it's taken out of the light. In other light-sensitive glass, the chemical must be heated for the reaction to reverse.
The problem with today's light-sensitive glass is that halogens react with organic molecules to make toxins and carcinogens. This makes recycling the glass difficult.
The new light sensitive glass is made from silver nitrate. It changes from clear to yellow after several minutes of exposure to an ultraviolet laser. It goes clear again after 15 minutes at 500 C.
Light sensitive glass isn't just for sunglasses, though. The researchers say their material could be used in optical computing as well, to store information using lasers.