Why do mirrors flip horizontally (but not vertically)?

Why do mirrors appear to flip images horizontally but not vertically?

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Featuring guest Meg Chetwood @megatronvolley

Thanks to Kyle Kitzmiller
and Veritasium http://youtube.com/veritasium

Music: "Rhythm Changes," "Locally Sourced"

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Closed Caption:


Have you ever
wondered why mirrors
appear to flip things
horizontally but not
vertically?
For example, this piece of
paper that says food, now
says doof in the mirror.
With backwards
letters, of course,
but they're still right side up.
How would the mirror know the x
direction from the y direction?
Another example, if I
raise my right hand,
the mirrored me
raises her left hand.
But if I put my
hand up in the air,
she also puts her
hand up, not down.
So what's going on?
My first knee jerk
response is that our eyes
are horizontal in our heads.
But the problem with this hunch
is that if you close one eye
and look in the mirror--
go ahead and try it--
the text still reads backwards.
Let's look again
at the word food.
When I read it, it's
facing me and I see "food."
But when I turn it toward
the mirror-- wait a minute.
I turn it.
The mirror is
reflecting it backwards
because I am presenting
it backwards.
Think of it this way, if I look
at the word, it is facing me.
When I look at it in the
mirror, I turn it away from me.
And in doing so, I
flip it horizontally.
That's why it
appears horizontally
flipped or backwards.
What if instead I flipped
it away from me like this?
That's a vertical flip.
And now the word is
flipped upside down.
It still reads "food"--
F-O-O-D-- in the correct order,
but now the letters
are vertically flipped.
So if I'm doing the flipping,
then the mirror is not.
Take an arrow and
point it to the right.
The reflection in the
mirror goes to the right.
Point it up, it goes up.
Left, it goes left.
Down, it goes down.
Forward, it goes--
wait a minute.
If I point the arrow forward,
the reflection goes backward.
So the mirror is
flipping the image,
but it's flipping it
in the z direction.
That's what's causing
all of this confusion.
When you look at your
reflection in the mirror,
it almost looks like
you've walked around
to the other side of the mirror
and you're facing yourself.
Hey, Meg.
Hey.
What side is my right hand on?
My left side.
What side is my left hand on?
My right side.
Perfect.
Just like a mirror.
But there's one problem.
Let's take a closer look
at our mirrored hands.
Mine says R.
But mine says L.
In a mirror image, her
hand would say R backwards.
To get Meg's hands
to look like this,
I'd have to flip her
image horizontally.
But we said that mirrors
don't flip horizontally.
So that's where the flip in
the z direction comes in.
If I could take my
body and flip it
through itself in
the z direction,
I would get my mirror image.
I know this is easier
to see with an arrow,
but I can also show it
with a left-handed glove.
If I hold it out
in the z direction,
flipping it front to back
would be the same thing
as turning inside out.
Then, it would be a
right-handed glove.
And it would fit on the right
hand of the me in the mirror.
All I did was flip the
glove front to back.
So why do we think things
are horizontally flipped
when they're really not?
We'll consider our
daily experiences.
Most of the words we see in
the mirror are on t-shirts.
We don't think of them
as horizontally flipped.
But that's exactly what
we do when we put them on.
We flip them away from our
normal reading position.
Another part of the
problem is that most of us
are left and right symmetric.
That causes you to see your
reflection in the mirror
as a normal looking
person facing you.
To face yourself
without the mirror,
you'd have to turn around.
And that's a horizontal flip.
It's hard to see what's
wrong with this image
unless you look closely.
But if you turn to face
yourself by doing a handstand,
that's a vertical flip.
And it's pretty obvious
what's wrong with this image,
because we're not
vertically symmetric.
So to answer the
question, why do mirrors
appear to flip
things horizontally?
Because you flip
them horizontally.
So the mirror is just showing
you what you presented to it.
Thanks for watching.
[THEME MUSIC]