Transition lenses are dual-purpose lenses that go from regular lenses indoors to sunglasses when exposed to sunlight outdoors. They are intended to provide corrective lens wearers with the benefit of not having to switch between their regular lenses and sunglasses.
Transition lenses are designed to be completely transparent under regular indoor lighting conditions. When the wearer exposes the lenses to any amount of UV light, the lenses will adjust correspondingly to the amount of light. For example, on a bright but cloudy day, the lenses might only darken halfway.
The main advantage of transition lenses is convenience. Transition lenses will adapt to any current lighting conditions to ease strain on your eyes. They keep you from needing to buy clunky clip-on or regular prescription sunglasses and can provide a convenient hands-free option, especially while driving.
One disadvantage is that occasionally they might not transition all the way to a transparent state indoors if the room is especially bright. Another disadvantage is that transition lenses do not always get as dark as regular sunglasses and might not provide enough protection for sensitive eyes. Also, transition lenses do not automatically change tints, instead taking several minutes to go from light to dark or dark to light.