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Name: Tay
Status: student
Grade: 6-8
Country: USA
Date: Winter 2013-14

Some sunglasses are clear indoors, but darken in sunlight. What ionic compound causes the lenses to do this?


Photochromic sunglasses, glasses that change color depending on exposure to UV light depend on a substances that change physical or chemical characteristics - and consequently their ability to absorb light. In lenses made of glass the most commonly used substance is a halide of silver (AgCl mostly). In the ionic state, Ag(+), does not absorb visible light appreciably, whereas silver metal, Ag, does. The process of transforming from Ag(+) to Ag is reversible and fast, so the sunglasses darken appreciably with UV-A exposure. In plastic lenses, a dye that absorbs UV and changes chemical structure, sometimes breaking bonds, sometimes changing physical arrangement, is used. The two different structures have different abilities or range of wavelengths that they absorb.

Greg (Roberto Gregorius) Canisius College

HI Tay,

Photochromic glasses, such as you describe, contain a layer of glass or plastic containing ultraviolet-light-sensitive silver halide compounds (such as silver chloride and silver bromide). These compounds are similar to the silver halide compounds found in black-and-white film.

Regards, Bob Wilson

Hi Tay, the dyes that make darkening glasses are special organic molecules that change their shape when exposed to ultraviolet light. Technically, they are not ionic. Note, the molecule itself changes shape, but the glasses/lenses stay the same shape. Because of the way these specially designed molecules absorb light, they absorb more after they have changed shape. When the UV light is removed, the dye relaxes back to its original shape, and in that original shape, it is not dark-colored. There are many more details you can learn if you are interested -- search for "photochromic dye" on the Internet and you can find lots more.

Hope this helps, Burr Zimmerman


It depends on the materials used to make the sunglasses. Originally, glasses had lenses made from glass used silver halide. The material was invented by Corning, and essentially worked in a similar manner to the film used in photography--light releases the silver and darkens the material. In lenses, the process is reversible, and in film, a chemical is used to freeze the darkening process (called "fixing"). In plastic lenses, more complex organic dyes are used, and exposure to light causes the molecules to change to a form that absorbs light.

Kyle J. Bunch, PhD, PE

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