According to Eyeglass Guide, Transitions lenses work because they contain certain photochromic dyes that react when they are exposed to the ultraviolet rays in sunlight, causing the eyeglass lenses to activate, or become darker. When the UV light diminishes, the photochromic dyes again react and cause the tint in the lenses to become lighter. As light levels change during the day, the tint in the lenses continues to adjust.
Eyeglass Guide emphasizes that it is important to know that Transitions lenses continue to provide a constant level of blockage of UV radiation for the eyes throughout the day, despite the changing tint levels due to varying amounts of light. The protection from harmful UV radiation, including both UVA and UVB rays, is constant and automatic. Since Transitions lenses react quickly to bright sunlight, they are also effective in reducing glare.
In other words, Eyeglass Guide explains that the photochromic dyes found in Transitions lenses get darker when the wearer goes outside. The lenses become clear once more when the wearer returns inside. It is important to understand that, since the photochromic dyes in Transitions lenses can be affected by very high temperatures, the lenses may not always get as dark as standard sunglasses.