Why does a car’s rearview mirror have a night setting?
The short answer
A reflection is light that bounces off a surface. The night setting reduces the brightness of headlights that bounce off the mirror. In a manual mirror, a small lever tilts the glass so less of that strong reflected light reaches your eyes. The headlights themselves do not get dimmer; the mirror sends less of their light toward you.
In the daytime, the mirror uses a strong reflection
Many manual rearview mirrors contain a wedge-shaped piece of glass. One edge is a little thicker than the other. A thin coating on the back surface reflects light well. In the daytime position, you mainly see the bright image from that coated surface.
The front surface of the glass also reflects a little light, but its image is much fainter. In daylight, the stronger image from the coated surface is easier to see.
The night setting moves the strong reflection away
When you move the lever, the glass tilts slightly. The strong reflection from the coated back surface now travels away from your eyes, toward the ceiling or another part of the car. Your eyes mainly see the faint reflection from the front surface instead.
The front and back surfaces are not parallel, so tilting the mirror changes which reflected light reaches you. The view behind the car looks dimmer, and the glare from headlights is reduced.
Some mirrors dim themselves
Some cars have an automatic-dimming mirror instead of a lever. Sensors detect bright light from behind, and an electronic layer in the mirror darkens to reduce the reflection. The exact sensors and controls vary by vehicle.
A dimmer mirror also shows a dimmer rear view
The night setting darkens the whole view in the mirror, including the road and vehicles behind you. It can make those details harder to see. Return a manual mirror to its daytime setting when glare is not a problem. Automatic mirrors work differently, so follow your car’s owner’s manual. Adjust the mirror while parked.
Key points
- A manual night setting changes the mirror’s angle and reduces the reflection that reaches your eyes.
- The headlights stay just as bright; their image in the mirror becomes dimmer.
- The view behind you also gets darker, so the daytime setting gives a clearer image when glare is not a problem.