Regular sunglasses block out particular wavelengths of light with either coloured filters or polarisation. As photochromic lenses are carbon-based, the molecules react to UV; they change shape and absorb the light, meaning the lenses start to look darker. The more UV rays there are, the darker the lenses will become. They adapt accordingly from clear through to different darker shades, depending on the levels of UV. The darkening can take up to 30 seconds for the tint to take effect on the glasses, and it can take between two to five minutes to return to normal when going back indoors.
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