Clever light-sensing technology allows mirrors in some modern cars to dim at night to avoid bright headlights behind from dazzling you. There is a sensor at the back of the interior mirror, it can sense darkness outside when it’s close to night time (or a dark tunnel etc). There is another sensor at the front of the mirror inside or close to the glass which sense how bright headlights behind you are. The anti-dazzle function of the mirror will automatically start working if needed. In  some cars this will also dim the door mirrors.