A photochromic lens contains light-reactive molecules (typically silver halide molecules or special organic compounds).
These molecules stay transparent in low light, but once exposed to UV rays, they change structure and darken the lens.

In short:

• Low light → clear lens

• Bright light → dark lens

All of this happens automatically. No lens swap. Nothing to touch.
Once UV intensity drops (clouds, shade, forest, end of day), the molecules return to their original form and the lens clears.

Modern technology makes this process fast. Often just seconds. That means stable vision in environments where light changes constantly, like the mountains or on a trail.