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Do you know about photochromatic sunglasses? Many guys have ever asked me about this question, as I am a glasses retailer, they thought that I must know much about all glasses lenses, actually, these so called sunglasses or prescription sunglasses can get darken when exposed to the sun, why does this happened? The reverse chemical reaction takes place when you go indoors and out of the UV light. The sudden absence of UV radiation causes the molecules to snap back to their original shape, resulting in the loss of their light absorbing properties. In either direction, the entire process occurs very rapidly.