edit: Thank you to all who answered! I’m amazed at how many ways you came up with to answer this.
It doesn’t.
It only flips front and back.
That’s the neat part - it doesn’t flip either. It’s your mind playing tricks on you.
Left and right, and the act of turning around are so common to you - that when you look at your reflection in the mirror, your brain expects that image to have turned around 180 degrees either left or right. Since that didn’t happen you think it’s “flipped”. But it’s not - your EXPECTATION is that it should be flipped.
Here’s another way to think about it. If it was common for us as humans to turn around by doing a handstand to look backwards - then you’d be complaining “why do mirrors flip up and down but not left and right?” But because that’s ridiculous and we don’t do that - you have no expectations that your mirror image should be standing on its head.
Trippy right? :)
Mirrors can totally reverse top-to-bottom, you just have to bend over to see it. The left-right bias is based on the way we look behind us, not any property of the mirror.
This takes a little explaining.
A rotation is a reversal through two dimensions at once.
If you turn around to look behind you, you’re swapping front-and-back, AND left-and-right.
If you stand on your head, you’re swapping front-and-back AND top-and-bottom.
Stand facing the way the mirror does, then turn to look into it. You have to do some kind of rotation - a two-dimension reversal - to get there. If you’re a normal human, you’ll twist around, swapping left-and-right as you swap back-and-front. Your left and right ear swap places, your nose and the back of your head swap places too.
But your reflection doesn’t do that.
A mirror only reverses ONE dimension: front-and-back. It’s the equivalent of punching your face out the back of your head: its ears are still on their original sides. You have swapped left and right in order to face in the opposite direction, but your reflection hasn’t - so it’s ears are on opposite sides to yours.
But you can do it the other way.
Stand with your back to the mirror, and bend over and look under your arm (or between your legs) to see your reflection, instead of twisting around.
Hold something with writing on it, and you’ll see: the letters in the reflection are upside-down, but they face in the right direction.
The only reason you don’t see this very often is that it’s a fucking weird thing to do and nobody ever does it.
Stand in front of the mirror and raise your right hand. In your imagination, trace the rays of light bouncing off of your hand, bouncing off the mirror, and entering your eyes. Your right hand is in front of the right half of the mirror (as you look at it) not the left half.
Your head, however, is on the top half of the mirror as you look at it, and your torso is on the lower part. The light is simply bouncing off the mirror at angles that align left, right, top and bottom relative to you.
The image of the human in the mirror appears to be raising the left hand, because your mind has re-oriented that person to assign “left” to that hand. But it’s really just the image of the right hand coming relatively straight back at you. It appears to be the person’s “left” hand because we think of the image as a person facing towards us. But it’s not a person. It’s just an image of you.
If it helps, move your right hand towards the mirror until it “touches the other guy’s left hand” and then back it off. Do this a few times and it should click what’s going on.
All of this assumes you are standing right side up. :0)
