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.