What Does That Tab On Your Rear View Mirror Really Do?

Of all the low-tech pieces of safety equipment that don’t get enough respect, perhaps none suffers quite as much from the Rodney Dangerfield treatment as the rear view mirror. This humble hunk of glass, metal and plastic may not look like much, but it’s responsible for your view out of an entire quarter of the car. It’s the key to knowing whether there’s a cop coming up behind you, a semi truck bearing down on you or whether your camping trailer has mysteriously vanished.

Of course, rearward visibility is something of a double-edged sword. You need light to flow through the back window and bounce off that reflective surface to see, but that same photon pathway is open to headlight beams, as well. Firing those intensive blasts into your sensitive rods and cones can leave you stunned or partially blinded — especially at night, when your pupils are wide open to soak in as much light as possible.

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Considering the slight impediment that a loss of vision might cause when piloting several thousand pounds of metal at 100 feet per second, carmakers have found a way to prevent it from happening (or at least reduce the effect): a dimmer switch, usually taking the form of a small tab at the bottom of the mirror. Push it, and the strength of the reflection is reduced, keeping the lights from slugging your eyeballs with extreme prejudice.

But how, exactly, does that little tab on the rear view mirror do that?

Well, it all comes down to how a rear view mirror is constructed. The reflective surface has two primary parts: the reflective surface of the mirror itself, and a piece of glass that covers it. Contrary to what you might expect, however, the glass isn’t of uniform thickness; it’s actually thicker at the top and thinner at the bottom. (Or, to put it geometrically: rather than run parallel to the surface of the mirror, the surface of the glass is at a slight acute angle to it.)

Because of this, the light that’s heading towards the driver is reflected not once, but twice — primarily off the highly reflective mirrored surface, but also off the less-reflective surface of the glass. And those two reflections reach the driver at two different elevations, with the weaker glass-sourced one below the stronger one from the mirrored surface.