It sounds like this“Mirrors seem to flip left and right because our eyes are side by side, not one above the other.”
Blaming our side-by-side eyes
Why it feels right
It is a neat and plausible explanation. Our eyes sit side by side, so we have a built-in horizontal axis; surely our brains notice horizontal swaps and ignore vertical ones. The explanation has the right sound: the fact has something to do with how we are built.
Why it’s wrong
The eyes are not the cause. Close one eye and the image still looks left-right reversed, and a camera records the same reflection. The reversal depends on the comparison we make: we imagine turning ourselves round, and turning swaps left and right.
A worked example
Test it. Close one eye and look in a mirror: the image still looks reversed left to right. Use a camera and look at the photo of a mirror: still the same. Lie on your side in front of a mirror with your head to the left: your reflection's head is also on the left, and if it seemed to flip, you would expect it to flip your head to the right. Nothing about two eyes explains that.
The real cause is how we make the comparison. Stand in front of a person facing you, and you see that their right hand is on your left. A mirror image looks like that person, but with something strange. It is easy to picture them as having turned around, and turning around about an upright axis swaps left and right. You turn yourself, not the mirror.
If we were used to turning around a horizontal axis, doing a handstand-flip, we would picture the image as upside down instead. The mirror itself does the same thing to all directions: it reverses only front to back. What changes is the way we imagine matching ourselves to the image.
How to catch yourself
- You say it is because our eyes are side by side.
- You believe the effect is a trick of human vision.
- You expect a camera to see something different.
What to hold instead
The left-right impression is not caused by our eyes. It comes from imagining the reflection as a person who has turned around about a vertical axis. The mirror reverses only front to back, and a camera records the same thing.
Where you’ll meet it
- Explaining why one-eyed people see the same reversal.
- Debating the question at a dinner party.
- Understanding why a photo of a mirror still looks reversed.
Questions people ask
- Is it because our eyes are side by side?
- No. The reversal looks the same with one eye shut and in a camera's photograph. It comes from how we mentally compare the image with a person who has turned to face us.
- Would a mirror look different to an alien with eyes above each other?
- Not in what the mirror does: it reverses front and back for everyone. What would change is the comparison we make in our heads, since someone used to turning about a horizontal axis would picture an upside-down swap.