A photochromic lens contains light-sensitive molecules (often silver halide molecules or special organic compounds).
These molecules remain transparent in low light, but as soon as they are exposed to UV rays, they change structure, causing the lens to darken.

In short:

• Low light → clear lens

• Bright light → dark lens

And all of this happens automatically, without you having to change the lens or touch anything.
Once UV intensity decreases (due to clouds, shade, a forest, or the end of the day), the molecules return to their original form and the lens becomes clear again.

Modern technology makes this process quick—often taking just a few seconds—ensuring stable vision in environments where lighting conditions are constantly changing, such as in the mountains or on a trail.