Photochromic lenses or photochromatic lenses are eyeglass lenses that are clear (or nearly clear) when used indoors but darken automatically when exposed to sunlight.
Modern photochromic lenses that tend to be plastic, contain carbon-based (organic) molecules called naphthopyrans in place of silver chemicals. These molecules react to light in a slightly different manner: they change their molecular structure in a subtle way when exposed to ultraviolet light. In this altered form, they have a different absorption spectrum and they soak up more ordinary light as it tries to pass by. This darkens the lenses.
Imagine many molecules darkening suddenly inside a clear lens. It’s like you close the blinds in front of your window on a sunny day. They progressively block out more and more light as the slats turn.
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Photochromic lenses take approximately a minute to darken and much more time to clear again. A French startup, Juic-e has developed a new lens technology that can make this happen in just 0.8 seconds. The technology does not need any battery and the system is completely automatic.
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