How Photochromic Glasses Work: The Science Behind Lenses That Darken in Sunlight

To protect our eyes from the sun’s strong rays, many of us wear sunglasses that filter out ultraviolet (UV) radiation, which can be harmful, and make it easier to see in bright, glaring light. However, those who need prescription glasses often face a challenge: when they go outside, they need to switch to a different pair, such as prescription sunglasses or clip-on tinted lenses.

A few years ago, an innovation changed this situation with the introduction of revolutionary glasses: indoors, they function like regular clear lenses, but when exposed to sunlight, the lenses darken and turn into sunglasses.

How Does It Work?

These darkening lenses, known professionally as photochromic lenses, are based on a discovery made in 1950 by Israeli chemist Yehuda Hirschberg. Hirschberg worked in the field of photochemistry, a branch of chemistry that studies how light waves affect atoms and compounds. He discovered that certain chemical compounds change color when they absorb ultraviolet light.

Some photochromic materials change color permanently when exposed to UV rays and are used in applications such as preventing sun damage, security features to prevent document forgery, and various research purposes. Others, however, change color when exposed to UV light but return to their original state once the light is removed.

From Clear to Dark — and Back Again

Photochromic glasses are made using these reversible materials. The lenses are coated with substances that remain nearly transparent in the absence of UV light. When exposed to sunlight, the materials react almost immediately, and within about a minute, the lenses darken into sunglasses. When you return indoors, the lenses gradually lighten until they return to their original clear state.

Photochromic materials are not limited to eyewear. They are also used in toys that change color during play, and even in cosmetics, where they respond to light exposure.