Dear Dsconrad,
Your deceptively simple question requires a very long answer, so I'll
refer you to the sources that have already been quoted. But I'll give
you a few pointers to help.
By "written backwards" you mean reversed left-to-right, right? Well,
the first point to notice is that the mirror didn't do this - _you_
did, when you turned the writing around to face the mirror! What the
mirror did was to reverse the scene from front to back. To understand
this, notice that the writing you are holding up to the mirror is in
reality invisible, since it is now on the far side of the paper. What
the mirror has done is pull the writing through the paper from the
back to the front so that it is now visible from the front. But it
hasn't changed the writing at all: the bit in your left hand (the end
of the writing) is still on the left, and the bit in your right hand
(the beginning of the writing) is still on the right - nothing has
changed. What has changed, of course, is that, having been pulled
through the paper like this, the writing is indeed enantiomorphic and
therefore unreadable.
You will find all this easier to investigate and understand if you try
the problem with a three-dimensional object instead of a
two-dimensional one. More simply, just put the writing on a
transparent medium such as glass or cellophane. Hold the writing up
to the mirror in the same way as you did before. Watch yourself
create the reversal as you turn the sheet to face the mirror. As you
look at the mirror image, look also at the original in your hands, now
visible through the back of the sheet. It, too, is "reversed" in just
the same way; in fact, the original and the image are exact copies,
point for point, over their whole extent.
Even better, try holding up this sheet to the mirror without first
turning it around to face the mirror. What can you see on the sheet
itself? It looks normal, of course, as you are now looking at its
front. And what do you see in the mirror? Why, the image is normal
too! As before, there has been no reversal. (I bet you won't believe
this last example until you try it!)
You can look at the problem a different way. If mirrors reversed from
left to right, they would need to know which way was horizontal, and
vertically-mounted mirrors don't. If you bought a rectangular mirror
for your home, you would need to specify whether you wanted to hang it
in portrait or landscape orientation, and you would get some very
curious effects if you hung it the wrong way. All this doesn't
happen, of course. No: the only direction a mirror knows about (and
this is true for any flat surface) is the direction _perpendicular_ to
its plane, and this is the only direction along which it could
possibly reverse anything. And that's exactly what it does.
Final hint: this is a complex issue of three-dimensional
understanding, so don't expect to grasp everything at a first reading
or first experimentation. Persevere and it will all suddenly become
clear.
I trust this helps.
Carnegie |
