What does the switch at the bottom of the rearview mirror do?

Car mirrors provide the necessary visibility to the rear and sides of the car. However, they can also be a source of discomfort - the glare from the headlights behind you through the rearview mirror is not pleasant and reduces your safety on the road. Luckily, the mirror is easy to adjust with a switch at the bottom of the rearview mirror.

What does the switch do?

If you have a manual rear view mirror, there is a switch or tab at the bottom. It must move up and down. Changing the position of the switch changes the way the mirror works. Flip it to one side and you're in daytime driving mode where everything is crisp and clear. Flip it the other way and it will switch to night driving mode. The reflection is dimmer (and harder to see when it's light outside), but it's designed for nighttime driving and cuts down on the glare from headlights behind you.

How Switches Work

So how does the mirror switch actually work? It's pretty simple, really. The glass in your rearview mirror isn't actually flat - it's a wedge of glass with one end thicker than the other. When you flip the switch at the bottom of the rearview mirror, the wedge moves. This changes how light passes through it and how it reflects back.

In daytime driving mode, the rear surface of the mirror reflects light and images. When you flip a switch and change the orientation of the mirrored glass, the front is responsible for what you see. Since light and images must first pass through the back of the glass before reaching the front and back to you, the image becomes dimmer and the glare from the headlights behind you is greatly reduced.