A mirror flips left and right, but not up and down

Hold a book in your hands while standing in front of a mirror. Now turn the book around so you can read it in the mirror. The text is backwards. But how did you rotate the book? Sideways? If you had flipped it over vertically, the text would be upside down instead of backwards. You are the one who reversed the book, not the mirror.

Re: A mirror flips left and right, but not up and down

#5

Post by bauchidgw » Thu, Jul 21, 2011, 6:58 PM UTC

i never understood the question: how could anybody ever even think that a mirror flips left and right? there is no emperical way to observe this behaviour (with a single flat mirror).

Re: A mirror flips left and right, but not up and down

#6

Post by JonnieCache » Thu, Jul 21, 2011, 7:02 PM UTC

The various feynman "fun to imagine" videos, one of which is linked by the answerer, are amazing. I advise you to watch all of them as soon as you can. They're all up on youtube as linked on stackexchange, or you can see them on the BBC's site here: http://www.bbc.co.uk/archive/feynman/

Also, feynman's explanation is much more intuitive and more easily understood than the one given on stackexchange. As you might imagine. Also it comes replete with the inevitable feynman tales of intellectual dickswinging that we have all come to know and love.

Re: A mirror flips left and right, but not up and down

#7

Post by andrenotgiant » Thu, Jul 21, 2011, 7:02 PM UTC

nice try, feynman

Re: A mirror flips left and right, but not up and down

#8

Post by InclinedPlane » Thu, Jul 21, 2011, 7:03 PM UTC

Mirror's don't flip left and right. We implicitly compare a mirror image to the original by rotating the object, and naturally we maintain top/bottom orientation, this gives the appearance of "flipping" the object left to right. However, if you use vertical rotation to super-impose the object on its mirror image you'll see that it seems to have been flipped top-to-bottom instead.

Re: A mirror flips left and right, but not up and down

#9

Post by andrenotgiant » Thu, Jul 21, 2011, 7:06 PM UTC

bauchidgw wrote: post #5

i never understood the question: how could anybody ever even think that a mirror flips left and right? there is no emperical way to observe this behaviour (with a single flat mirror).

This is one of those Feynman questions where understanding the question requires a certain way of thinking that discards common sense. Similar to his "Why is the wall not see-through?" question.

The way I think about is: "When you are looking at a mirror, and you blink your left eye, mirror-you blinks his right eye. But when you are laying on your side looking at a mirror, and you blink your top eye, mirror you blinks his top eye."

Re: A mirror flips left and right, but not up and down

#10

Post by InclinedPlane » Thu, Jul 21, 2011, 7:09 PM UTC

bauchidgw wrote: post #5

i never understood the question: how could anybody ever even think that a mirror flips left and right? there is no emperical way to observe this behaviour (with a single flat mirror).

If there were a person who looked like a version of you who was "flipped" left/right they would look exactly like the mirror version of you. However, it's equally valid to consider such a "mirror-person" as being flipped top/bottom or inverted front to back. However, these are much less intuitive transforms.