Q: How Does the Rearview Mirror Work for Nighttime driving?

The answer to that depends on which type of rearview mirror you have. If you’re thinking of the rearview mirrors we’ve been using for decades, with the little tab on the bottom that flips up or down, let’s talk about them first. Then we’ll cover the newer automatic dimming type.

When we look into a rear view mirror we’re actually looking through a piece of glass that has a mirror mounted at an angle behind it. In the daytime setting, we get full reflection directly toward our eyes. Flipping the tab selects nighttime mode by changing the angle of the mirror. This angle only allows the mirror to reflect a portion of the light coming at us from the glaring headlights behind us. We still see an accurate reflected image, just dimmer one.

• Tip: The adjusting tab, or lever, is a two-position switch. Move it all the way one way or the other, but not in between.

Automatic dimming rear view mirrors actually darken the tint of the mirror glass in response to the brightness of the headlights shining in from behind. They utilize a technology called electrochromic. This “smart glass” has a thin coating applied and is wired to a controller that sends a low-voltage current to automatically lighten or darken the tint of the glass as required. The mirror may have an on-off switch on the side or bottom to activate this feature. This same technology as being used in “smart windows” of office buildings to lower the heat load from the sun as it moves throughout the day.