STEM Around Us: Partial sunglasses – Photochromic lenses

Sun through spectacles, Credit: Wikimedia/Thuwaraga vilvanathan

One of the most harmful components of sunlight is its UV rays. Apart from multiple skin diseases, UV rays can also cause serious damage to our eyes. Chronic exposure is known to decrease vision and induce cataracts. Reducing UV ray exposure is very important. One can do just that with the help of photochromic lenses. In the presence of UV light the lenses turn black like those of sunglasses, while in the absence of UV light the lenses go clear.

A chemical called silver chloride absorbs UV rays and changes shape, which causes it to undergo a chemical reaction, darkening the glass. The darkened glass blocks UV rays. Without UV rays, the reaction does not occur. If the glasses were already darkened, a reverse reaction occurs that makes the glasses transparent again. Silver chloride starts to react within a minute, but takes a further 10 minutes to completely darken the glass. Although photochromic lenses are budget-friendly, their activity is affected by changes in temperature. Also, they are not dark enough for people facing severe light-related eye issues.

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