How Does the Rear View Mirror Dimmer Work?

So you're driving down the highway at night and listening to some tunes when all of the sudden someone comes up on your tail end. Next thing you know their headlights are blaring into the car, blinding you as they reflect off the rear view mirror. Thinking quickly, you flip the little switch under the mirror, tinting the reflection and preventing you from driving blindly.

So how does the simple flip of a switch decrease the reflectiveness of the mirror? It turns out that even though there is only one piece of glass making up the mirror, it actually has two reflecting surfaces that are out of parallel with each other; they look like a wedge from the side view.

side view: |/

The back surface is covered with silver which gives it strong reflectivity while the front surface is not coated at all (but still has some reflective ability).

Daytime

During the day the silver pane will be facing the road behind you while the un-coated pane will be pointing at the back seats.

|/ <-- incoming light from the road hits highly reflective surface

** back seat **

The silver pane acts as a perfect mirror giving you a good view of what's happening on the road while the un-coated pane will reflect very little off the back seats.

Night

At night the situation is different. Now the road is very dark and headlights will reflect off the silver surface strongly. The solution is to flip the switch so the un-coated pane faces towards the road while the silver pane faces at the ceiling.

** ceiling **

\| <-- incoming light from the road road hits less-reflective surface

The un-coated side has similar reflectivity to when you can see yourself in a window's reflection in your house. This surface reflects enough to tell you what's going on on the road behind you without blinding you at the same time.

Overlapping reflections?

You may sometimes notice two different reflections in the mirror. This happens because the reflective surface that isn't facing the road is still reflecting whatever it's pointing at. The reflection is usually too dim to be distinguishable, but not all the time. For example, if you set the rear view mirror to the night mode and shine a light at the ceiling you will see two reflections:

• one of the road behind you from the un-coated pane, and
• one of the light on the ceiling being reflected off the silver pane

The future

Many new cars are being equipped with rear view mirrors that dim automatically using electronic systems that detect incoming light and react accordingly to keep the glare at a minimum. While these systems are innovative, the simplicity of the wedge-shaped mirror and its effectiveness make it much more impressive in my book.