Physics
Psychology

Why do mirrors reverse only left and right but not up and down too?

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2013-01-29 14:18:56
2013-01-29 14:18:56

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.

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