The substances responsible for photochromatic property of a lens are microcrystalline silver halides (usually silver chloride). These substances embed into glass version of lenses to make them photochromatic. Photochromatic lens contains millions of silver halides crystals. When exposed to sunlight, UV rays transform the crystals into light-absorbing particles. As these particles absorb light, the lens darkens and the amount of light passing through the lens is reduced.
When the lenses are removed from sunlight and the stimulating UV radiation is no longer present, the crystals return to their original orientation and the lenses become clear.
Plastic lenses depends upon organic photochromatic molecules for their reversible darkening. The main reason these lenses darken in the sunlight and not in indoors is that indoor does not contain ultraviolet rays. The Photochromatic lenses come back to their original color due to thermal process. Because of this reason some lenses don’t work well in hot temperatures.
More on Photochromatic lenses.
The transition lenses are today most popular photochromatic lenses. Transition lenses are manufactured by transition optical. Transitions applies a thin layer of photochromatic particles in front of the surface of lenses instead of distributing light-sensitive particles throughout the lens material. The light sensitive material penetrate lens evenly and up to depth of 0.15 millimetres.