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

Pushing the tab on a car's rearview mirror seems to dim the headlights of the car behind you -- but how? Learn how rearview mirrors work in this classic episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, production of I Heart Radio. Hey
brain Stuff, I'm Lauren boge Obam and this is another
classic episode from our erstwhile host, Christian Sager. Today's topic
features a bit of everyday science, well every day for
anyone who drives. What do those tabs on rear view
mirrors actually do? Hey, I'm Christian Sager and welcome to

(00:27):
brain Stuff. If you have a car, then chances are
you also have a rear view mirror. So imagine that
one night you're driving home when some joker rolls up
behind you with his high beams on, blinding you. Luckily,
you're able to flip the tab on your rear view
and make the mirror glare resistant, eliminating most of the light.
But what's going on there? How can the same piece
of glass have two different amounts of reflection? This mirror's

(00:51):
looks can be deceiving. See, your rear view isn't just
one mirror, and it isn't flat. Instead, it's built in
what's called a prismatic edge shape, and it contains two
reflective surfaces. There's a regular shiny mirror in the back
and there's a thin glass wedge right in front of
it closer to the driver. The front glass surface is

(01:12):
at an angle to the back surface. If you were
to look at this mirror outside of its casing, it
would be wedge shaped with the thicker edge up at
the top. During a daytime drive, the angled glass surface
is pointing down and you're observing the reflection of the
main mirror. But when you flip that little tab, the
back mirrord surface usually points towards the dark ceiling, you

(01:34):
don't see that image. What you see instead is the
image reflecting off the front of the glass. The glass
only reflects about four percent of the light hitting the mirror,
so it doesn't hurt your eyes. To prove that this
is what's happening, take a flashlight with you one night
and play with your mirror. Now, not while you're driving,
but you know, while you stopped. Flip the mirror into

(01:55):
glare resistant mode and shine the light at the ceiling
or sometimes try the floor. The fully reflected image will
overwhelm the front surface reflections, so you can see the ceiling.
This is just the mechanism behind the basic rear view mirror.
More high end mirrors may have auto dimming electrochromic technology,
which uses a low voltage power supply to tint the glass,

(02:18):
or you might even drive a car with a rear
view camera system. Today's episode was written by Christian and
produced by Tyler Clang. Brain Stuff is a production of
iHeart Radio's How Stuff Works from Oneness and lots of
other topics with more than one angle, visit our home
planet how stuff Works dot com. Plus for more podcasts

(02:38):
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or wherever you listen to your favorite shows.