In order to avoid the kind of glare from your rear view mirror that can seriously impair your night vision, automakers harnessed the latest technology. Certain substances, called electrochromic materials, can darken in color when an electrical charge is applied to them. The power of these materials is brought into play by sandwiching some electrochromic gel between two pieces of glass in your rear view mirror housing. A sensor on the housing detects the amount of light coming from behind. When it’s bright enough, the unit fires an electric charge into the gel, causing it to darken and absorb excess light so it doesn’t shine in your eyes.
Once the vehicle behind you turns or otherwise leaves your lane, the sensor directs the electric charge to end, the gel returns to its usual color, and your mirror reflects the normal nighttime scene behind you.