Why does a mirror appear to invert the left-right directions, but not up-down?
physics/mirror Added material
Problem
Why does a mirror appear to invert the left-right directions, but not up-down?
Show solution
Mirrors invert front to back, not left to right. The popular misconception of the inversion is caused by the fact that a person when looking at another person expects him/her to face her/him, so with the left-hand side to the right. When facing oneself (in the mirror) one sees an 'uninverted' person. See Martin Gardner, ``Hexaflexagons and other mathematical diversions,'' University of Chicago Press 1988, Chapter 16. A letter by R.D. Tschigi and J.L. Taylor published in this book states that the fundamental reason is: ``Human beings are superficially and grossly bilaterally symmetrical, but subjectively and behaviorally they are relatively asymmetrical. The very fact that we can distinguish our right from our left side implies an asymettry of the perceiving system, as noted by Ernst Mach in 1900. We are thus, to a certain extent, an asymmetrical mind dwelling in a bilaterally symmetrical body, at least with respect to a casual visual inspection of our external form.'' Martin Gardner has also written the book ``The Ambidextrous Universe.''
Additional material
Mike Naylor wrote: >Speaking of mirrors, why do they reverse left and right, but not up >and down? Presumably Mike knows the answer to this already (or at least the address to the FAQ). The answer in the FAQ, while correct, does not (IMHO) satisfy people. It doesn't always give the necessary insight into the situation to understand what is going on. I prefer the following: Picture a mirror placed on a ceiling. It reverses up and down. Often this is enough to get the person thinking and to understand. If it isn't, I usually suggest that they picture a 3-D object (say orthogonally crossed red, yellow, and blue arrows) inside a cubic room with mirrors on each of the walls. At this point, if it still isn't clear, well, so far its always been clear. This may be the most commonly asked rec.puzzles FAQ question that I hear outside this newsgroup. -- Matt <mroozee@math.uci.edu>, October 1997 Or maybe its that the light sticks to the mirror in much the same way as if you had painted yourself and walked into the mirror. [ Editor: this last line follows the signature in the same posting. ]