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The Science Behind the Dimmer Switch on a Rear-View Mirror

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dimmer switch on a rear-view mirror button night driving activate

Photo: The News Wheel

There’s a lot of cool things that your car does without you understanding how it works — and we’re not just talking about everything that happens under the hood. Even the smallest convenience features took inspiration and ingenuity to create.

Take the rear-view mirror for instance. Have you ever wondered how the dimmer switch on a rear-view mirror works? Here’s the clever, simple secret to how it reduces the brightness of headlights behind you while still allowing you to see.

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A simple explanation of the dimmer switch on a rear-view mirror

HowStuffWorks does a great job explaining the science behind the dimmer switch on a rear-view mirror.

Your car’s rear-view mirror isn’t one single unit; it’s made up of components, namely a wedge-shaped mirror and a pane of glass covering that mirror. When the mirror is in its normal position, light rays that hit your eyes — allowing you to see what’s behind you — have been reflected from their source onto the mirror.

All that tab does is slightly lean the angle of the mirror back so that it doesn’t reflect directly into your eyes. Instead, the reflection will bounce above your head instead of down to your eyes.

How are you still able to see behind you, then? The less-bright reflection you’re now seeing isn’t coming from the mirror; it’s bouncing off the glass pane. Most of the light rays are still passing through the glass and bouncing off the mirror, so only a small fraction of the light is now reaching your eyes.

An auto-dimming mirror, though, works a little differently. It uses camera and light sensors to send charges to a gel in between panes of glass to limit how much light it lets through to reflect off the mirror.

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Sources: HowStuffWorks, Jalopnik