Photochromic glasses, also known as transition lenses, are eyeglass lenses that darken when exposed to ultraviolet (UV) light from the sun and return to a clear state when indoors or away from UV light. They are made of various lens materials, including plastic, glass, or polycarbonate. The lenses use photochromic molecules that react to light and change their structure, causing the lens to darken. The first photochromic lenses were patented in the 1960s and were made of glass, while plastic versions followed in the 1980s and 1990s and are now the more popular choice among glasses wearers.

Photochromic lenses are ideal for people who need to wear glasses all the time, as they provide the benefits of glasses and sunglasses in a single frame. They are used to protect the eyes from UV light and eliminate the need for a separate pair of prescription sunglasses. They are also a great help for those who always wear eyeglasses, as they reduce the need for two separate pairs of prescription eyewear.

Photochromic lenses darken significantly within about a minute of exposure to bright light and take somewhat longer to clear. They are principally used in glasses that are dark in bright sunlight but clear, or more rarely, lightly tinted in low ambient light conditions. The lenses are available in a range of clear and dark transmittances.

In summary, photochromic glasses are eyeglass lenses that darken when exposed to UV light from the sun and return to a clear state when indoors or away from UV light. They are ideal for people who need to wear glasses all the time and eliminate the need for a separate pair of prescription sunglasses.