Photochromic lenses contain light-sensitive molecules. Outdoors, ultraviolet radiation triggers a change in their structure that increases absorption of visible light, so the lens becomes darker. When the ultraviolet exposure drops, the molecules return toward a clearer state.
A reversible molecular change
The effect is chemical and reversible, but it is not an instant electronic switch. Different formulations and temperatures influence how quickly a lens darkens and clears. The lens is responding chiefly to ultraviolet exposure, not simply to whether a scene looks bright to a person.
Why the response varies
That explains a familiar surprise: many car windshields filter much of the ultraviolet light, so some photochromic lenses darken less behind them even on a sunny day. Product formulations vary, and ordinary clear lenses can have UV protection without becoming dark. Polarized sunglasses tackle a different problem by filtering light with a particular orientation to reduce glare.
Related: how polarized sunglasses reduce glare.