A photochromic lens contains light-reactive molecules (often silver halide molecules or special organic compounds).
These molecules remain transparent when there is little light, but as soon as they are exposed to UV rays, they change structure, making the lens darker.

Brief :

• Low light → clear lens

• Lots of light → dark lens

And all of this happens automatically, without you needing to change lenses or touch anything.
Once the intensity of UV light decreases (clouds, shade, forest, end of day), the molecules return to their initial shape and the lens becomes clear again.

Modern technology makes this process quick, often in just a few seconds, which ensures stable vision in environments where the light is constantly changing, such as in the mountains or on a trail.