Why does a mirror reverse left and right, but not up and down? and does it have something to do with us having two eyes?

Physics girl explains it well: https://m.youtube.com/watch?v=vBpxhfBlVLU

This explanation makes sense to me until (as seppel mentioned below) I think of a webcam. A webcam (minus any software) isn’t flipped at all—but looks like a mirror if you flip it along the x axis, as many programs do by default.

Take a camera and point it at a friend, now turn it around and point it at yourself.

How did you turn the camera?

You could have turned it towards you by turning it bottom over top and then you would have to vertically flip the image, but you probably would have never even thought to do it.

So instead you chose to turn it left to right so to get a mirror image you have to flip the pixels left to right.

You’re human so flipping a camera upside down seems quite silly but that’s just an artifact of your environment.

That was a really good explanation. In particular the part that we don’t flip left-right, but rather forward-backward and how she illustrated it with the glove.

A mirror reverses front and back. Because we are (more or less) horizontally symmetrical, we reduce the discrepancy by rotating the image around a vertical axis, resulting in a left-right reversal. If we were vertically symmetrical we would see mirrors as reversing up and down; if we were front-back symmetrical (a pushmi-pullyu) we would see mirrors as they are.

It's not reversing those, it's reversing the forward.

Look in a mirror at what's behind you, then rotate your head upwards until you're looking at the behind. Look back at the mirror.

As you can see, the mirror is reversing up and down, not left and right.

Because a mirror doesn't reverse anything. The only reversed part is our perception because we compare it to how a person actually standing in front of us would look.

> Because a mirror doesn't reverse anything.

Well, a mirror gives you a different picture than a webcam and a computer screen (unless the webcam software flips the picture along the x axis).

Now why is that, that the software needs to flip along the x axis? It is actually because a mirror mirrors around the the z axis, so it switches front and back.

It's the webcam that is flipped.

The software of the webcam usually flips the video, so that it looks like a mirror to you.

That's not true, that would mean that webcam video by default would flip any writing and make it unreadable.

> That's not true, that would mean that webcam video by default would flip any writing and make it unreadable.

That is exactly what happens if I open Facetime on my Mac and hold something in the camera.

To be a bit more precise: There are two streams generated. One is shown to yourself, and this one works like a mirror. The other one is shown to remote participants and this one is not flipped.

Many do, to you the person holding the writing, but not to the people you are showing it to.

Draw a straight line from every point on your body onto a wall and that's a mirror. To reverse up and down you would have to draw those lines in weird ways.

A mirror is not reversing right and left. If you point to the right(East) your mirror image is also pointing to your right(East).

Because when you turned from facing away from the mirror to facing towards it, you rotated around the z axis and not the x axis (assuming a coordinate system where x is horizontal, y is into the mirror, and z is up).