? What is the substance called that makes my glasses turn dark in sunlight ?



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    These particular glasses are photochromatic.  The lenses undergo a chemical reaction when exposed to ultraviolet radiation.  The chemical reaction causes the chemicals in the photochromatic lenses to change shape.  This new shape absorbs the visible UV rays and the lenses become dark.  When the eye glasses are no longer exposed to the ultraviolet radiation (the sun) the molecules return to their former shape, losing their light absorbing properties, they become light again. This process can only happen in natural sunlight. 

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