Why do mirrors reverse left and right but not up and down?
In: Physics
The mirror doesn’t flip anything. It reflects exactly what it sees.
You are just used to seeing things flipped left-right because that is what happens in the real world when you face another human. They rotate 180 degrees on the Y axis to look at you.
Wear a shirt with some words on the front. Bend sideways at the hip so you’re facing the mirror with your torso sideways. The words on your shirt will now be “up and down reversed.”
The mirror will always reflect light so that the angle of light arriving (“angle of incidence”) will equal the angle of light reflection.
With pieces of yarn and some tape, you can even mark off the 3-D geometry between the letters on your shirt and the reflection in the mirror to show that there’s no geometrical left-right tricks going on.
Some people sum that up by saying “You were the one who swapped the letters left-right, when you put the T-shirt on. You started off looking at the face of the shirt to read it, then you flipped it around before putting it over your head.”
They don’t.
The way you think about it is that if you want to get to that position, you’d turn around – essentially walking around an imaginary vertical pole. But why a vertical pole? Why not a horizontal pole? If you flipped over instead, you’d be doing a handstand. You’d be the opposite way up from the image, but left and right would be the same.
The mirror would reverse up and down if you stood on it or if you put it on the ceiling above your head
None of these is correct.
The Answer is that mirrors invert things on the Z-axis, not X or Y. That is, they reverse things forwards and backwards. They *invert* rather than “flip.”
Here’s an excellent explanation.
They don’t reverse anything left to right (or up and down).
You’re thinking of a word printed on a shirt, and when you stand in front of the mirror it looks ‘backwards’ to you. It’s your perception that makes it seem as though it’s flipped around, but it’s actually you who is flipped around relative to the shirt.
To prove to yourself that things aren’t actually flipped around, think about the rear-view mirror in a car. When you look in the mirror the driver in the car behind you is still on the driver’s side of the car (i.e. on the left side of the mirror, if you live in most of the Americas or continental Europe).
All the mirror is really reversing is *depth* relative to the mirror. Things that are closest to the mirror appear closer to you, in the mirror, than things that are further from the mirror.
Here’s a fun little experiment you can try at home, if you have something clear like a transparent plastic sheet to write on: write a word on that sheet, then hold it in front of you so that you can read it. It will appear in the exact same orientation in the mirror, and the mirror image will be easily readable to you too. Flip it around, as though you were wearing it on a shirt, and you’ll quickly see why words look ‘backwards’ in mirrors.
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