Well, our world, only slightly different.

Like, you’ve probably noticed that words look backwards in a mirror, but they aren’t

upside down - which is weird, because how would a mirror know to flip left and right

but not up and down?

Well, the short, annoying answer is that mirrors don’t flip left and right OR up and down

- if you look carefully at a scene in a mirror, you’ll notice that everything on the left

stays on the left, everything on the right stays on the right, everything up stays up,

and everything down stays down.

What does get flipped is the direction into or out of the mirror - the things closest

to us end up farthest away, and the things farthest from us end up closest.

Mirrors flip not left and right, nor up and down, but depth - they flip in and out.

And mirrors invert in and out because they reflect light according to the principle of

specular reflection: light coming into the mirror at a given angle bounces back out at

the same angle.

This means that, after bouncing off a mirror, light is behaving no differently than it would

have if instead of a mirror there were a window into a parallel universe on the other side,

just like ours in every way except with in and out reversed.

That’s why mirrors feel like windows into another world - the light is behaving in LITERALLY

the same way as it would if they WERE windows into another world.

Ok, but then what about the left-right flip question?

How does that make sense if mirrors DON’T flip left and right?

Well, you can literally see the answer for yourself by writing on something transparent

- when the word is readable left-to-right, its mirror image is also readable left to

right.

The reason words are usually flipped in mirrors isn’t because mirrors flip them; WE flip

them.

Words, it turns out, tend to be printed on or attached to objects that we have to turn

to face the mirror in order to see in the mirror.

WE flip the words - and we normally do it left & right.

We could just as well turn them vertically, in which case the word would look flipped

top-bottom in the mirror, not left-right!

Again, not the mirror’s doing.

Here’s the deal: if a word is flipped in a mirror, it’s probably flipped outside

the mirror, too!

We just don’t normally notice that the word got flipped outside because we can’t see

through solid objects.

In order to see the word without looking in the mirror, we have to go around to the other

side of the object and turn ourselves around.

Unless you have multiple mirrors - in that case, in and out get flipped along two different

directions, which combines to mean that things on the left indeed show up on the right and

right on the left - or, if we rotate the mirrors, up gets flipped with down.

So if somebody tells you mirrors don’t flip left and right - well, mirrORS actually CAN

- but not mirrOR.

A lone, single mirror has to get us to do the flipping for it.

Hey, before you flip channels, I’ve got a channel recommendation for you: MinuteBody.

Wait, but I thought this video was sponsored by CuriosityStream, which has teamed up with

Nebula, a new streaming service featuring original videos from independent and education-y

youtube creators?

Well yes it is - and those original series include MinuteBody, produced by the MinuteEarth

team, or “The Logistics of D-Day” produced by Real Engineering.

And you can get Nebula for free bundled with a CuriosityStream subscription by going to

curiositystream.com/minutephysics, which gets you access to the thousands of documentaries

on CuriosityStream AND all of Nebula for just $20 a year.

So go to curiositystream.com/minutephysics and you can watch Particle Fever, MinuteBody,

AND the logistics of D-day.

Oh, and new MinutePhysics videos are there too, in case you want to watch without ads.

Thanks to CuriosityStream and Nebula for supporting educational videos on the internet!