Colour-changing glasses are a fascinating magic trick: indoors, the lenses are transparent, but when exposed to sunlight, they turn coloured.
Colour-changing glasses are made of photochromic glass. The magician hidden inside the photochromic glass is silver halide (silver bromide or silver chloride) and trace amounts of copper oxide. These two compounds are added to the raw materials used to make photochromic glass and then melted. When exposed to strong sunlight outdoors, the silver halide microcrystals in the photochromic glass decompose under the ultraviolet rays of the sun, releasing silver atoms. When the glass is filled with silver microparticles, the lenses become darker. Returning indoors from sunlight, where the indoor ultraviolet rays are extremely weak, under the catalysis of copper oxide as a catalyst, the silver atoms react with halogen compounds to form silver halide again, making the lenses colourless and transparent. The chemical reaction that occurs here is: colour-changing glasses use this chemical principle to make the lenses change colour.