Re: Fabulous Adventures In Coding (Eric Lippert)

From: Al Dunbar [MS-MVP] (alan-no-drub-spam_at_hotmail.com)
Date: 02/16/04 

Date: Sun, 15 Feb 2004 20:09:48 -0700

"John Ford" <zjcf@DawnAndJohn.net> wrote in message
news:eifOdtC9DHA.4060@tk2msftngp13.phx.gbl...
> On 2/13/2004 12:50 AM Al Dunbar [MS-MVP] wrote:
>
> > "Richard Cornford" <Richard@litotes.demon.co.uk> wrote in message
> > news:c0haec$pd1$1$8302bc10@news.demon.co.uk...
> >
> >>"Al Dunbar [MS-MVP]" <alan-no-drub-spam@hotmail.com> wrote in message
> >>news:eQOj9sS8DHA.1040@TK2MSFTNGP10.phx.gbl...
> >><snip>
> >>
> >>>But seriously, now, can Eric tell us why things seen in a
> >>>mirror are reversed side-to-side but not top-to-bottom?
> <snip>
> > What, did nobody point out that this defect all mirrors have can be
easily
> > fixed? All you have to do is turn out the light, and the mirror will no
> > longer reverse things any which way.
> >
> > /Al
>
> Forgive my late entry into this foray.

Forgiven. But you might be too late to get a mark for your answer.

> I humbly (OK, not so humbly)
> submit that the mirror does not reverse anything. Up is still up, right
> is still right, etc.. Our expectation is backward, because we most
> commonly turn around to see something that is behind us. When we do
> that, WE have reversed OUR right and left. Since that's what we're used
> to, we assume that a mirror reverses the image. But it only shows the
> reverse of what we expect.

Put another way, we commonly use our own orientation as the common frame of
reference. However in a few cases, and mainly out of expediency, we have
realized that it is easier to consider that it is we who have turned and not
the rest of the universe. If we consistently stayed with one frame of
reference or the other, we would likely be less confused about the issue.

> When two people face each other and one says "move to the right", you
> must know whose "right", because one of you is reversed.

Only the one who says actually knows for sure what is meant; for the
listener, it's a crapshoot. If it were you and me facing off this way, I
would say "move to your right" to avoid ambiguity. This could still fail if
you were dyslexic, but that is a risk I would take. If I said "move to my
right" what would you do? Some would come and stand beside me, others would
ask what I wanted them to do, but the majority would tell me that I am not
the boss of everything.

Now, back to our standoff, what would you do if, instead of saying "move to
your right", I said "move from my left"?

> If you both
> face the same direction and see each other in a mirror, there is no
> right/left issue. If we were accustomed to bending over backward to look
> behind us (thus reversing up and down), then when the person you're
> looking at says "move to the right", it's the same direction for both of
> you.

Sorry, I cannot move sideways while waiting for paramedics to arrive to put
me into traction.

> The concept of up and down is not nearly as ambiguous as right and left
> in our day-to-day lives, thanks mainly to gravity.

Possibly true for most of us. But fighter pilots work within a gravitational
field and have as much concern for up and down as left and right. Perhaps it
is not the direct effect of gravity that makes up/down less ambiguous, but
the indirect result of it generally constraining our movements to the
horizontal.

> Thus, when we arrange
> ourselves and a mirror such that up and down might appear reversed, we
> intuitively know that it is either us or the object, not the object's
> image, that is reversed from normal.

Unless, of course, we are upside down for too long and pass out while
pondering the question.

/Al

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