Photosensitive lenses: oxidation-reduction reactions

Reactions in which electrons are lost or gained are called redox reactions. They are very important in our daily lives, they are present in several technological inventions, and it is based on them that we can explain the chemistry involved in the photosynthetic lenses of sunglasses. Sun.
It all starts with the composition of photochromic glass: when tetrahedral oxygen atoms bond with silicon, a crystalline structure of silver chloride appears. This structure is disordered, so that there are gaps between the atoms, thus, visible light passes through this structure.
The advantages of lenses made with photochromic glass is that they do not let ultraviolet light through, they absorb this light, and an oxidation-reduction reaction between silver and chlorine ions arises. See the equation:
Ag⁺ + Cl^- → Ass²⁺ + Cl^-
Through this reaction, silver chloride crystals are formed, but so that the reaction does not become reversible, Cu+ ions are added. Follow the reaction:
Ass⁺ + Cl⁰ → Cu²⁺ + Cl^-
Note that the Cu ions

⁺ reacted with the chlorine atoms formed in the previous reaction. The biggest question is: how does the lens darken in the presence of light? Colloidal metallic silver forms on the surface of the lens, it has the property of absorbing light, which makes the lens dark to protect the eyes from ultraviolet light.
But why when we go back to the dark environment the lenses clear again? Because the chloride ions present in the crystal of the photochromic glass rejoin silver ions through molecular rearrangements.
Now you know why photosensitive lenses are best for good eye health: blocking ultraviolet rays.

By Líria Alves
Graduated in Chemistry

Source: Brazil School - https://brasilescola.uol.com.br/quimica/lentes-fotossensiveis-reacoes-oxirreducao.htm