It reflects your image across the plane of the mirror. This isn’t physically possible with real-world objects, so your brain substitutes the closest real-world transformation that would approximate the same appearance—which is rotating bilaterally symmetric objects about their axis of symmetry. With a vertically-oriented human, that means turning around so your left is on the right and vice versa.

Mirror images more resemble being flipped inside out. Sort of.

Imaging holding your hand out flat, parallel to the ground while facing a mirror. The tips of your fingers would be closest to the mirror.

Now imagine if you were wearing a glove on that hand and you took the glove off so it were inside out.

If you were to compare the inside out glove to the reflection of the hand the glove came from, they would look similar.

Mirrors just show us what they see.

Let Dr Feynman guide you through this question.

It depends on how you want to think about it. I think Action Lab did the best short video explanation of how your perception of mirrors makes things look flipped.

https://www.youtube.com/watch?v=bDpWhg-hbHo

What are you talking about?

Well, because it doesn’t flip anything. The light just bounces off, thus light from your right bounces back to your right.

The mirror DOESN’T flip left and right. Imagine an x-axis as a horizontal line across the mirror, a y-axis as a vertical line up/down the mirror, and a z-axis as a line that comes straight out of/into the mirror.

The mirror is actually only flipping that z-axis that comes out of/into the mirror, by reflecting the light back

Tilt your head 90 degrees to one side. Bam! Now it flips up and down.

I got you:

https://www.youtube.com/watch?v=8b7Kp2uBUZg

It doesn’t. It actually flips front and back.

Because the mirror flips your eyes!

You got a left and a right eye. Now if you had a top and a bottom eye instead, then the mirror would not flip left and right, but top and bottom.

Buuut… if you had 4 eyes: left, right, top and bottom, then the mirror would flip them all!

Because it’s not flipping, it’s reflecting.

When you face someone, why does your left and their left flipped from one another ?
Any object we face are flipped : it’s true also where you are in front of a car from instance … it doesn’t happen when you are inside the car or inside yourself.
if you can see this, you can understand the mirror as well.

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)

It does. Every part of a reflection is made so that your reflection touches you. If it were upside down, it couldn’t do that. If both you and your reflection raised your left hand instead of one raising their right hand, it also couldn’t do that, and there would be nothing preventing you from reaching your hand in that parallel universe.

I am an aphantasiac and needed to look at the two queen of hearts in a deck of cards to realize this.