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Why does a mirror reverse things horizontally but not vertically?
Mirror mirror on the wall, who is the most laterally inverted of them all?
Why, you are! Whoever you are looking into me because the left hand side of your face
now appears on the right and that boil that mysteriously migrated from one cheek to the
other. You have been reversed, but only in the horizontal plane.
Mirror mirror on the wall, you have you only reversed me in that plane? Why isn't my head
upside down as well? You're only a flat sheet of reflecting glass, albeit a talking one.
This cussed quality of mirrors that they only reverse things in one direction but not the
other is something we grasp from a very early age. No sooner have children learned to write
cat and dog, than they work out how to write bum backwards on a piece of paper, and then
look at it in the mirror the right way around. It's hilarious.
It's nothing to do with the mirror itself, which doesn't care if it's held upside down
or if you hold the piece of paper on its side. Or if you lie on your side but hold the piece
of paper horizontal to the ground, M, U and a back-to-front B, will always come out as
BUM when viewed in the mirror. But it will never be upside down.
Some people imagine that the answer to this is something to do with having left and right
eyes rather than having top and bottom eyes. But actually that makes absolutely no difference,
if you look in the mirror with one eye shut then the spot on your right cheek is still
on the left cheek of the person in the reflection who looks exactly like you but the other way
round compared with that photograph you have of yourself. The photograph is actually part
of the explanation. The camera sees you as another person would with your right on their
left, but the mirror doesn't do this it presents your right on your right.
In fact the mirror image is more like an old fashioned transparency photograph viewed the
wrong way round. As an image your face is two dimensional if it were partly transparent
as well, then the view of it in the mirror would be the same as you looking at it from
behind. Let's return to a piece of paper, this time
with BUM written on it the right way round. Now if you write that on a very thin piece
of paper, and instead of holding it up to a mirror you hold it up to a light, you will
see M, U and back-to-front B. Because the mirror doesn't reverse left and right it actually
reverses front to back. Or to put it another way, our left is also
the mirror's left, but because the mirror reverses everything it thinks of that as right.
Right? Confusing? Writing comes out on a mirror back
to front because we actually present it to the mirror back to front, so back to front
writing comes out in the mirror as the right way around.
If a mirror could read it would have learned to read back to front, but if it could talk
it would talk the right way around because it might reverse your ears but it doesn't
reverse sound or time. Fortunately.