WEBVTT - BrainStuff Classics: What Does the Tab on a Rearview Mirror Really Do?

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<v Speaker 1>Welcome to brain Stuff, production of I Heart Radio. Hey

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<v Speaker 1>brain Stuff, I'm Lauren boge Obam and this is another

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<v Speaker 1>classic episode from our erstwhile host, Christian Sager. Today's topic

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<v Speaker 1>features a bit of everyday science, well every day for

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<v Speaker 1>anyone who drives. What do those tabs on rear view

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<v Speaker 1>mirrors actually do? Hey, I'm Christian Sager and welcome to

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<v Speaker 1>brain Stuff. If you have a car, then chances are

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<v Speaker 1>you also have a rear view mirror. So imagine that

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<v Speaker 1>one night you're driving home when some joker rolls up

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<v Speaker 1>behind you with his high beams on, blinding you. Luckily,

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<v Speaker 1>you're able to flip the tab on your rear view

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<v Speaker 1>and make the mirror glare resistant, eliminating most of the light.

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<v Speaker 1>But what's going on there? How can the same piece

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<v Speaker 1>of glass have two different amounts of reflection? This mirror's

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<v Speaker 1>looks can be deceiving. See, your rear view isn't just

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<v Speaker 1>one mirror, and it isn't flat. Instead, it's built in

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<v Speaker 1>what's called a prismatic edge shape, and it contains two

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<v Speaker 1>reflective surfaces. There's a regular shiny mirror in the back

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<v Speaker 1>and there's a thin glass wedge right in front of

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<v Speaker 1>it closer to the driver. The front glass surface is

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<v Speaker 1>at an angle to the back surface. If you were

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<v Speaker 1>to look at this mirror outside of its casing, it

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<v Speaker 1>would be wedge shaped with the thicker edge up at

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<v Speaker 1>the top. During a daytime drive, the angled glass surface

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<v Speaker 1>is pointing down and you're observing the reflection of the

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<v Speaker 1>main mirror. But when you flip that little tab, the

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<v Speaker 1>back mirrord surface usually points towards the dark ceiling, you

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<v Speaker 1>don't see that image. What you see instead is the

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<v Speaker 1>image reflecting off the front of the glass. The glass

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<v Speaker 1>only reflects about four percent of the light hitting the mirror,

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<v Speaker 1>so it doesn't hurt your eyes. To prove that this

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<v Speaker 1>is what's happening, take a flashlight with you one night

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<v Speaker 1>and play with your mirror. Now, not while you're driving,

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<v Speaker 1>but you know, while you stopped. Flip the mirror into

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<v Speaker 1>glare resistant mode and shine the light at the ceiling

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<v Speaker 1>or sometimes try the floor. The fully reflected image will

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<v Speaker 1>overwhelm the front surface reflections, so you can see the ceiling.

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<v Speaker 1>This is just the mechanism behind the basic rear view mirror.

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<v Speaker 1>More high end mirrors may have auto dimming electrochromic technology,

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<v Speaker 1>which uses a low voltage power supply to tint the glass,

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<v Speaker 1>or you might even drive a car with a rear

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<v Speaker 1>view camera system. Today's episode was written by Christian and

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<v Speaker 1>produced by Tyler Clang. Brain Stuff is a production of

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<v Speaker 1>iHeart Radio's How Stuff Works from Oneness and lots of

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<v Speaker 1>other topics with more than one angle, visit our home

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<v Speaker 1>planet how stuff Works dot com. Plus for more podcasts

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<v Speaker 1>for my heart Radio, visit the iHeart Radio app, Apple Podcasts,

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