A mirror reverses your reflection so that your left and right hands
are interchanged but your head and feet are not. Even if you lie on
your side and look at your reflection your head and feet remain where
you'd expect them but your left and right hands are swapped. What is
the most succinct way of completely explaining this strange behaviour.
I don't want ray diagrams, I want simple words of English! Thanks

Hi, playitagainsam-ga:

"Why does a mirror reverse left and right, but not top and bottom?"

It doesn't.

The mirror reverses front and back, not left and right (nor top and
bottom).

We are accustomed to having someone or something which is facing away
from us "turn around" to see the front (instead of the back).  It is
actually the act of turning around which reverses left and right.  The
mirror faithfully puts the left on the left and the right on the
right, which has come to seem "backwards" to us; we "expect" the left
to be on the right and conversely.

Perhaps if we customarily flipped upside down to face one another
(instead of turning around), our perception of the mirror image might
be that it "reverses up and down".

regards, mathtalk-ga

---

Request for Answer Clarification by playitagainsam-ga on 07 Jul 2003 09:16 PDT

Thanks, your answer is succinct which is great, and has set me
thinking again and so I am now imagining life in a space ship in which
my fellow astronauts find it easier to flip upaide down when they turn
around, and after 10 years of space flight I have come to be used to
this and every time I pick up an object to examine it I also flip it
on its horizontal rather than its vertical axis as well - I am pretty
used to this,  however one day I get a zit on my chin and in the
ship's washroom I look in the mirror, the zit appears to be on my left
cheek so I know to put the zit cream on my right cheek - after 10
years of space flight the darn mirror has done its sneaky left-right
flip again. Do you see what I am getting at?

---

Clarification of Answer by mathtalk-ga on 07 Jul 2003 11:34 PDT

To be honest, it seems that you simply assert the existence of a
left-right confusion and blame it on the mirror.  I've never
experienced this.  When grooming myself it seems abundantly clear that
to affect something which appears on the left side of my image in the
mirror I must do something to the left side of my face, and this is
correct.  (Of course if we were facing another person, rather than own
our image in the mirror, we should need care to remember our "right"
is their "left" and so forth.)

Where mirror images do get me confused has more to do with front to
back movements.  For example, when trying to shave the tufts of hair
around the back of my neck, I find it quite awkward to move my
electric razor in the proper direction while watching it in the
mirror.  If I stop watching in the mirror and simply think about
pulling the razor around the back of my neck, it's quite simple.  But
the mirror turns a "forward motion" into a backward one, and this I
find causes real consternation.

Curiously enough the image which is formed on our retinas is reversed
(both right/left and up/down) by the lens of the eye.  Our brains make
the perceptual interpretation come out correct.  Experiments with
goggle devices that reverse images (using prisms) have shown that in
roughly a month's time people can learn to "rewire" their brains and
compensate for this:

http://abcnews.go.com/sections/science/DailyNews/vision000922.html

regards, mathtalk-ga

Succinct and accurate and a great help - thanks!

Thanks for taking time to rate my answer, playitagainsam!  I enjoyed
your space station scenario of flipping end over to face one another. 
I confess that my suggestion that if we were accustomed to this, it
might lead to speaking of a mirror reversing top and bottom, is
slightly facetious.  Our bodies have a strong left-right symmetry but
no up-down symmetry.  Thus when an image is reversed (say in printing
a photograph "backwards"), we tend to focus on the difficulty of
detecting a left-right asymmetry in the image (e.g. looking for the
face of a clock rather than the face of a human).  This left-right
confusion is another factor in why we "suspect" the mirror as we do.

regards, mathtalk

Thanks again. I had suspected that this 'physics' question would
inevitably lead into areas of perception and human psychology. I
wonder what answer I would have got from a psychologist (I doubt it
would have been a better one). A final thought - Leonardo da Vinci's
mirror writing. If we want to read it using a mirror, I guess we have
to be careful to turn the paper to the mirror the correct way
otherwise it's upside down as well as back to front. Oh yes, we also
have to learn 17th century italian.

It also makes me think of the Belgian surrealist Rene' Magritte's 1937
painting, La Reproduction Interdite ("copying prohibited").  A man
stands looking into a mirror wherein is reflected not his face, but
the back of his head!

[Magritte's La Reproduction Interdite]
http://www.malekzadeh.com/art/images/mag_09.jpg

That what strikes one at first as merely an absurdity has a deeper
meaning has made Magritte (1898-1967) a celebrated artist.  For a
physicist's comment, see this by Antoine Weis of the University of
Fribourg:

[Atomic physics tests of the Standard Model]
http://www.unifr.ch/physics/frap/3cycle/Lecture1.pdf

"A mirror does not invert LEFT and RIGHT, but rather FRONT and BACK."
(page 18, with illustrations)

Incidentally the subject of La Reproduction Interdite is a real
person, Edward James.  For some background on this patron of the arts,
see this:

[Smithsonian Article]
http://www.junglegossip.com/smithmag.html


Search Strategy

Keywords:  Magritte "La Reproduction Interdite" (pages in English)
://www.google.com/search?q=Magritte+%22La+Reproduction+Interdite%22&hl=en&ie=UTF-8&oe=UTF-8&lr=lang_en&sa=X&oi=lrtip8

I might just be stupid, but...

Could the strange "left-right but not up-down" flip (or non-flip
depending on your diffinitions) arise from (or at least be related to)
the simple fact that our eyes are located side by side and not one
over the other? (And thus our visual cortex might make various
assumptions in this regard.)

While I must admit that I tend to accept the original answer as being
the correct one, it is interresting to note that when you slant your
head 90 degrees while looking in a mirror, you will no longer percieve
a left-right flip (assuming you keep left and right oriented relative
to the ground), whereas up and down will now be subject to said
"flipping"!

One thing that talks against this view is the fact that you still
experience "flipping" with one eye closed (although this could
possibly be dismissed with the above argument, namely that our visual
cortex was "designed" for use with two eyes, arranged side by side).

Well, just a thought (and you didn't even have to pay a penny!)

P.S. Perhaps your space-station story has even more to it: On earth it
would seem likely that our visual cortex is aided by the up-down
reference produced by gravity (by way of our sense of balance). In
this light it is amazing that one is able to see normally while in
space (or other zero-g environment) ;)