What Do Those Tabs On Rearview Mirrors Actually Do?

Sure, rearview mirrors are cool -- but how do they actually work? How can the same piece of glass have two different amounts of reflection? Join Christian as he explains the science behind these handy devices.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from How Stuff Works. Hey, I'm
Christian Sager, and welcome to 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

(00:24):
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 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 wedge shape, and

(00:45):
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 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

(01:06):
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 mirrored surface usually points towards the
dark ceiling. You 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

(01:29):
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 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

(01:51):
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. Or you might even
drive a car with a rear view camera system. Check
out the brainstuff channel on YouTube, and for more on

(02:13):
this and thousands of other topics, visit how stuff works
dot com.