Why do transition lenses go dark in sunlight when you’re outside but not when you’re in a brightly lit room?
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Transition lenses use most commonly silver chloride (AgCl) which, as many have pointed out, reacts with UV light. When hit by UV light, the molecules change shape and absorb a significant percentage of the visible light. This causes a grey discoloration, which has the effect of reducing light passing through, thus having the darkening effect. Most people who use it though will tell you that it has more of an effect of reducing glare and less of darkening in the way that normal shades do.
As others have also pointed out, the fact that it reacts with UV light is the reason why it doesn’t change colour in a bright room. Some light sources will trigger it though. I used to wear those and had fun triggering the darkening over those old overhead projectors that we used in classrooms before digital projection took over. You had to put it very close to the light source in order to do that though (like directly sitting on the OHP).
This change is reversible, so AgCl reverts back to its transparent state when in the absence of UV light.
Source: used to use them, and read up on it a long time ago when I first got them. Stopped using them because they got in the way of doing photography, which I enjoy as a hobby.
Edit: added in the last sentence of the first paragraph
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