What Is The Little Tab Under Your Car's Rear-View Mirror For?
The small tab under the rear-view mirror changes the tilt of the mirror so that the headlights of the cars behind you don't dazzle you at night. It works as the mirror's 'night mode' switch, yet many drivers still don't know what it is for.
The movable tab under the rear-view mirror is not a decoration.
Most people either never notice the small tab under or at the edge of the mirror, or assume it has no function. In fact, it directly affects comfort and safety when driving at night.
To be clear from the start: the tab is not for adjusting the mirror's viewing angle to your seating position. Once you have turned the mirror by hand and set its direction, you don't need to touch the tab. Its only job is to switch the mirror between its day and night positions.
Flip the tab and the mirror switches to night mode.
At night, the headlights of the car behind you reflect from the rear-view mirror straight into your eyes. That glare dazzles you, briefly disrupts your view of the road and tires you out.
In most cars, pulling the tab towards you puts the mirror in the night position. In some models the direction is reversed, so it is worth checking your owner's manual. In night mode, the mirror reflects the strong light from behind away from your eyes. The image looks a little dim, but you can still easily make out the vehicles behind you. Remember to return the tab to its original position in daylight, because night mode reduces clarity during the day.
The Glass Is Wedge-Shaped, Not Flat
The glass in a manual rear-view mirror is a wedge (a prism) whose two surfaces are not parallel. The back surface is silver-coated, and in the day position you get the bright, sharp image from that coating.
When you flip the tab, the mirror tilts slightly. The strong reflection from the back coating shifts up towards the roof, and what reaches your eyes is the much weaker reflection from the front surface of the glass. Because that reflection carries only a few percent of the light, the brightness of the headlights drops noticeably. So the mirror doesn't actually darken; it simply shows you a reflection that isn't as bright.
In cars without a tab, the mirror dims by itself.
Not every car has a tab. Auto-dimming mirrors have two light sensors, one facing forwards and one facing backwards. If the light coming from behind is too strong compared with the ambient light, electricity is applied to the electrochromic layer inside the glass and the mirror darkens by itself. You don't have to touch anything.
Many new cars come with this feature, but because the system is expensive, manual mirrors are still far more common.
Try It On Your Next Night Drive
If you have never used night mode, try it on your next night journey. The difference is clearest on long roads and outside the city, where vehicles with high beams come up behind you. Eye strain is reduced, and you avoid the loss of vision that follows glare.
Flipping the tab under the rear-view mirror takes a few seconds and gives you a more comfortable, safer night drive in return.
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