Question

why some spectacle lenses turn brown when sunlight falls on them? ​

Answer 1

Answer:

But 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. The number of molecules that change shape varies with the intensity of the UV rays.

Answer 2

Explanation:

Photochromic lenses use specific molecules that appear transparent in visible light and the absence of UV. When exposed to UV in sunlight, the molecules undergo a chemical change in shape and absorb parts of the visible spectrum, appearing to darken