Question

darkening property of photochromic glass is due to the presence of

Answer 1

Darkening property of the photo chromic glass is due to the presence of silver chloride

Answer 2

Photochromic glass uses special lenses which gets darker if exposed to certain intensity. These lenses can be made of plastic, glass or polycarbonate material.  

The material present in photochrome lenses which causes the darkening in certain intensity of light is silver chloride or silver halide.

They turn dark significantly within a minute of exposure to bright light and take some time to come back.