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Speaker 1: Welcome to brain stuff from How Stuff Works. Hey, I'm

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Speaker 1: Christian Sager, and welcome to brain Stuff. If you have

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Speaker 1: a car, then chances are you also have a rear

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Speaker 1: view mirror. So imagine that one night you're driving home

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Speaker 1: when some joker rolls up behind you with his high

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Speaker 1: beams on, blinding you. Luckily, you're able to flip the

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Speaker 1: tab on your rear view and make the mirror glare resistant,

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Speaker 1: eliminating most of the light. But what's going on there?

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Speaker 1: How can the same piece of glass have two different

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Speaker 1: amounts of reflection? This mirror's looks can be deceiving. See,

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Speaker 1: your rear view isn't just one mirror, and it isn't flat. Instead,

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Speaker 1: it's built in what's called a prismatic wedge shape, and

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Speaker 1: it contains two reflective surfaces. There's a regular shiny mirror

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Speaker 1: in the back, and there's a thin glass wedge right

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Speaker 1: in front of it, closer to the driver. The front

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Speaker 1: glass surface is at an angle to the back surface.

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Speaker 1: If you were to look at this mirror outside of

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Speaker 1: its casing, it would be wedge shaped, with the thicker

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Speaker 1: edge up at the top. During a daytime drive, the

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Speaker 1: angled glass surface is pointing down and you're observing the

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Speaker 1: reflection of the main mirror. But when you flip that

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Speaker 1: little tab, the back mirrored surface usually points towards the

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Speaker 1: dark ceiling. You don't see that image. What you see

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Speaker 1: instead is the image reflecting off the front of the glass.

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Speaker 1: The glass only reflects about four percent of the light

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Speaker 1: hitting the mirror, so it doesn't hurt your eyes. To

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Speaker 1: prove that this is what's happening, take a flashlight with

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Speaker 1: you one night and play with your mirror. Now not

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Speaker 1: while you're driving, but you know, while you stopped. Flip

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Speaker 1: the mirror into glare resistant mode and shine the light

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Speaker 1: at the ceiling or sometimes try the floor. The fully

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Speaker 1: reflected image will overwhelm the front surface reflections so you

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Speaker 1: can see the ceiling. This is just the mechanism behind

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Speaker 1: the basic rear view mirror. More high end mirrors may

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Speaker 1: have auto dimming electrochromic technology which uses a low voltage

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Speaker 1: power supply to tint the glass. Or you might even

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Speaker 1: drive a car with a rear view camera system. Check

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Speaker 1: out the brainstuff channel on YouTube, and for more on

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Speaker 1: this and thousands of other topics, visit how stuff works

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Speaker 1: dot com.