Photochromic lens. What are photochromic lenses?

At some point in our lives we have already visited an ophthalmologist. On this visit, we underwent several tests to assess our vision. If we are not seeing well, the doctor makes the diagnosis, identifying the type of visual impairment, and prescribes a specific prescription for the problem. If we look closely at a prescription prescribed by an ophthalmologist, we will see a series of numbers and abbreviations whose meaning is easily understood by the professional in the optical laboratory.
The most varied types of vision defect that we can present: myopia, farsightedness, astigmatism and presbyopia. Interestingly, there is a type of lens that can correct each of these defects. For example, myopia can be corrected using divergent lens, hyperopia is corrected using converging lens. Thus, the main purpose of studying Optics is the understanding of human vision, which by the way is quite complex.
As we know, not all people wear glasses because they have vision defects. Many wear them just for aesthetics or wear glasses that are intended to protect their eyesight from the sun. These glasses are popularly known as “dark glasses”. For those who wear prescription glasses, it can be said that it is a little more complicated to wear sunglasses, as many of them do not have the correct degree of visual correction needed by the person. That's where the calls come in

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photochromic lenses.
You've probably come across a person walking around on a sunny day wearing sunglasses that changed color when that person entered a dimly lit room. As we mentioned in the paragraph above, these lenses are called photochromic lenses.
At photochromic lenses have a film of three silver salts: silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI). In the presence of intense light, they form metallic silver, separated from the other elements. As this type of silver is opaque, the lens becomes dark. When the luminous intensity decreases, the metallic silver re-combines with the other elements, recovering the salts, and the lens becomes colorless again.

By Domitiano Marques
Graduated in Physics

Source: Brazil School - https://brasilescola.uol.com.br/fisica/lente-fotocromatica.htm

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