Rear-view Mirror Dimmer

I drive a sedan on the roadways of California.  I find the proliferation of trucks with jacked up lift kits and upgraded super-bright headlights in this state to be pretty obnoxious.  The towering stature of these road-monsters puts their headlights right in line with my rear-view mirror, which blasts my night vision when driving in the evening (Note: I personally think that these trucks, as vehicles, are awesome.  But another thing that annoys me about this situation is that most Southern Californians don’t even use them correctly–they buy them as mechanical-codpiece-status-symbols.  If I bought one, I’d actually run the truck through some mud once in a while.  The majority of Southern Californians don’t understand that that is what they are for.).

Fortunately for sedan owners, automotive and optics engineers are smart enough to supply a simple solution to this problem with most cars–the rear-view mirror dimmer.  In many cars, this is simply  a mechanical toggle switch at the base of the mirror.  But I was curious, how does this switch make the image behind you dim?

This rear-view mirror dimmer relies on two things:

1. The optical properties of “mirror glass” and “mirror silver”–namely: the silver has a much higher reflectivity value than the mirror glass, and,
2. The mirror silver surface and the mirror glass surface are not parallel.

In the “flipped-down position” of the toggle switch, the rear-view mirror functions as a typical mirror–objects behind your car are reflected to your eye off of the mirror silver backing:

But while in the “flipped-up position,” the mirror is rotated up and the plane of the silver mirror backing is oriented so that it no longer reflects the image into your eyes (instead, this image is reflected up to the ceiling of your car, or down to the floor, depending on the design). In the “dimmed” case, the image behind your car is reflected off of the mirror glass surface:

If you have seen the outline of your reflection in a clean glass surface, you know that the intensity of the image is very faint and this is exactly what optics engineers count on when designing the “dimming” feature of a rear-view mirror.  By reflecting the image to your eye off of the glass surface–which has much lower reflectivity–the jackass behind you is far less able to blind you.

Given the number of times I use this feature in my car, the “rear-view mirror dimmer” is a valuable mechanism without qualification.