What is the chemistry behind sunglasses?

People wear sunglasses throughout the year to block out the sun for comfort or health purposes. Many people who wear prescription glasses can even opt for photochromatic lenses that allow the convenience of sunglasses only when UV rays are present.

To have sunglasses that can change their tint, color and function, there's a significant amount of chemistry that goes into building this accessory. As sunglasses have evolved over the years, more chemistry has been involved. Whether you're a researcher interested in working with the chemical compounds used in sunglasses or just a curious sunglass user who wants to learn more, here's some interesting information about how chemicals help protect our eyes from the sun. 

Sunglasses tied to movie stars 
Many people think of big sunglasses as a glamorous, movie star accessory, but why? As glass giant Corning explained, movie stars actually popularized the use of sunglasses in the 1920s. They wore them to protect their eyes from the bright movie sets. Prior to the 1920s, the brightness of the sun was largely dealt with through hats, visors and evolution. Early sunglasses were simply tinted glass. Inventors toyed with tinted designs for corrective reasons, but sunglasses did not seriously develop until the 20th century. 

As more people began developing and improving sunglasses, different colors emerged for various purposes. Gray sunglasses protect well against glare but do not enhance contrast, while amber glass or plastic distort color, improving depth perception and contrast. 

What's in modern sunglasses? 
Whether the lens is glass or plastic, regular sunglasses are made of a variety of chemical compounds. Made How explained that metal oxide is a critical aspect of both plastic and glass lenses to protect against UV rays. Plastic also is coated with soluble organic dyes, while glass lenses are made with additional metal particles. 

The lenses are post-coated following the final shaping of each lens as well. This process involves the application of product-specific anti-reflective chemicals in a vacuum coating method. 

How photochromatic lenses work 
For many lens types, such as Corning PolarCoat sunglasses, the specific chemicals used are a closely guarded secret. But for photochromatic lenses, there have been a variety of chemicals used over the years that are well known. 

Initially, embedded silver halide crystals were mixed into the glass of the lenses to create photochromatic sunglasses, The Chemistry Blog explained. The reaction between the cations of the silver compound and the electrons of glass creates a tint in the presence of UV light. When indoors, there was no reaction and the glass stayed clear. 

Then plastic photochromatic lenses came onto the market featuring pyridobenzoxazines. As plastic lenses evolved, naphthopyrans and indenonaphthopyrans have been used. Indenonaphthopyrans are the current choice for plastic lenses and react to UV light by breaking their chemical bond. To respond to this broken bond, the chemical reforms darker and tints the lens, the blog explained. 

There is a debate among photochromatic sunglass users as to whether plastic or glass is better. Glass lenses are less popular but do last longer because they use inorganic silver chemicals versus organic compounds that can break down over time. Glass is also more resilient to normal wear and tear such as scratching. 

One of the biggest downsides of photchromatic lenses is also caused by another piece of chemically coated glass – the windshield. The polyvinyl butyral coating used to limit UV penetration on windshields makes photochromtic lenses ineffective while driving. 

The chemistry of sunglasses is still a fairly young science, with many convenient advances made in recent years. Although most users may not realize there is a chemical reaction occurring in front of their eyes when they walk outdoors into the sunlight, they definitely appreciate the safety that indenonaphthopyrans carry.