Why are left and right reversed in a mirror but not up and down?

Answer 1

Our bodies are fairly symmetrical about a vertical plane, i.e. the right side is an opposite image of the left. (If we fold ourselves along a vertical line from head to toes, then each bodily side accommodates the other.) This does not apply to the horizontal plane, where the head stands in opposition to our feet.

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Another way to imagine this is to see yourself in front of and facing a mirror, but lying on your left side. You can see that your left [bottom] and right [top] sides still seem "reversed" by the mirror. You can also see that your head and feet are not reversed. A mirror image is a little like walking into a wall of soft plaster or clay, leaving a 'negative' image of yourself.

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Comment: Here is the answer (without going into too many details).

This does answer the question.

A plane mirror reverses the object ONLY in the direction perpendicular to the mirror surface. That's ALL it does.

However people often perceive this as a left right reversal with front back unchanged. That's the "psychology of perception" not physics.

Answer 2

There are two key points to understanding why a mirror appears to flip images left/right and not up/down.

Key point 1: Mirrors actually flip images front/back. They do not flip images left/right OR up/down. Imagine holding an arrow-shaped cardboard cutout in front of a mirror. The side of the cardboard arrow facing you is painted white, the side facing the mirror is black. The arrow is pointing to the right. When you look at the actual arrow in your hand, you see a white arrow pointing to the right. When you look at the image of the arrow in the mirror, you see a black arrow pointing to the right. The mirror has not flipped the arrow from left to right, just from front to back (that is, it has allowed us to see the back of the object as if the back was the front, without changing either the up/down or left/right orientation. Note: If you don't have a good imagination, try this for real. It's important. (I didn't call it "Key Point 1" for nothing.)

Key point 2: The directions "left" and "right" are relative to the direction a person is facing. The directions "Up" and "Down" are not. So, when the mirror reverses a person's image front to back, the left/right point of reference changes, and it appears as if the mirror has reversed things left to right. But up/down is not affected by this change from front to back.

I know what you're thinking. "That makes sense for people, but why does the same left/right reversal happen when I view a page of writing in a mirror?"

Pause for those who want to figure it out for themselves before they read ahead.....

Answer: Because when you turned the page to face the mirror, you rotated it on the vertical axis. YOU reversed the letters left to right. All the mirror did was allow you to see the reversed letters. Try flipping the page on the horizontal axis instead (flip the page top to bottom), and the mirror will again let you see what you have done: reversed the letters and words top to bottom, but not left to right. (Note: You don't even need a mirror - flip a page and hold it up to a light so you can see through the paper to the words on the other side. You will see the same thing as the mirror shows you, proving that all a mirror does is allow us to see the back side of an object.

Comments: Obviously the "mirrors" under discussion are flat (plane mirrors).

I do think people often over complicate this question. Yes there's plenty of room for discussion of different examples, but the main thing is that people often perceive the "front back" reversal of a mirror as a left right reversal, as stated

in the answer above.

I'm familiar with the sort of arguments mentioned above about a page of writing. I prefer to treat "writing" as a three dimensional object with a front and a back. Then you don't need a separate argument for why writing appears reversed. It's just another object that's reversed only in the direction perpendicular to the mirror surface.

Answer 3

This is called "lateral inversion", lateral meaning "sideways". It happens, as you say, when you look into an ordinary flat mirror.

This effect is partly an illusion.

The only thing that the mirror does is to reverse you in the direction perpendicular to the mirror surface. So it doesn't really reverse either left and right or up and down. It treats them both the same.

So that gets rid of that problem, but leaves the problem of why left and right SEEM to be reversed.

It's to do with psychology. We find it easy to ignore the front to back reversal

and that means it looks like left and right are reversed.

Answer 4

Try looking into different types of mirrors such as a convex or concave mirrors.

Answer 5

The mirror is flipping objects horizontally, causing left to appear as right and vice versa. However, it does not flip objects vertically, which is why up and down remain unchanged.