Please note that this is only the coating and you must select a lens (Single vision or Varifocal) along with it
A Photochromic lens, or often called transition lens, becomes darker as it is exposed to UV, thus normally on exposure to sunlight. The in the absence of UV the lens will return to its clear form.
It is important to note that the lenses may not become as dark in a car as the windscreen and windows in a car have a UV filter thus hindering the reaction of the lens. On cloudy days during daytime the lenses may also still get a little bit darker as there will still be exposed to UV.
This type of lens approximately takes 90 seconds to go from being clear to dark in bight sun conditions and up to 120 seconds to change back to its clear form when going back indoors in the absence of UV.