1. Self-Darkening Eyeglasses By : Brittany Christian

2. Two-for-one Eyeglasses Two for one eyeglasse lenses that darken when exposed to ultraviolet rays from the sun and return to their clear state indoors have come a long way since they were first invented in the 1960s. The first photo chromic lenses were made of glass and developed by scientists in the U.S. at Corning. The company has made improvements to its original process over the years, but in general, the science remains the same.

3. Three different ways to look at the self darkening eyeglasses

4. Photochromic Glass photo chromic lenses contain silver halide crystals embedded in a glass substrate. In the presence of UV-A light (wavelengths of 320–400 nm), electrons from the glass combine with the colorless silver cations to form elemental silver. Because elemental silver is visible, the lenses appear darker. Back in the shade, this reaction is reversed. The silver returns to its original ionic state, and the lenses become clear.

5. Self darkening glasses when absorbing UV light. Self darkening glasses when absorbing UV and visible light.

6. The Chemistry The chemistry of plastic photo chromic lenses is completely different from that of glass. The first generation of plastic lenses relied on a family of organic dyes known as blue pyridobenzoxazines. Subsequent generations used naphthopyrans, and today, Transitions makes its lenses with indenonaphthopyrans. When a photo chromic dye is exposed to UV radiation, a chemical bond is broken. The molecule then rearranges into a species that absorbs at longer wavelengths in the visible region, causing the lens to darken. Improvements in the dyes over the years have led to faster activation and clearing, as well as longer lifetimes.