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What are photochromic glasses?

Photochromic lenses have millions of molecules of substances such as silver chloride or silver halide embedded in them.
When exposed to UV rays, as in direct sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken, and to improve vision while ensuring protection from hazardous optical radiation.
The darkening of photochromic lenses was reversible, i.e., the lenses became original shape again once the ambient light was lowered.

An important note about photochromic lenses:

because they react to UV light and not to visible light, there are circumstances under which the darkening will not occur. Frist example, is in your car. Because the windshield blocks out most UV light, photochromic lenses will not darken. Second,is indoors test with ordinary lighting(Because the lighting used in daily lighting does not contain ultraviolet light),photochromic lenses will not darken.
So the ways to activate photochromism:
Exposed to UV rays, as in direct sunlight OR Use ultraviolet light lamp (not ordinary lighting lamp).This is the way to activate the photochromic function of the product.