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From: throopw@rtp47.UUCP (Wayne Throop)
Newsgroups: net.physics
Subject: Re: Yet Another Flaky Paradox
Message-ID: <53@rtp47.UUCP>
Date: Thu, 30-May-85 10:38:58 EDT
Article-I.D.: rtp47.53
Posted: Thu May 30 10:38:58 1985
Date-Received: Sat, 1-Jun-85 14:42:22 EDT
References: <5377@tektronix.UUCP>
Organization: Data General, RTP, NC
Lines: 33

> This has more to do with clear thinking than with physics, but here goes:
>
> When you look in a mirror, the image you see is reversed left to right.
> Why is it not also reversed in the vertical direction?
>
> -- Carl

Well, actually, it isn't a paradox, and you don't need to think very clearly
either.   The usual phrase to ask the question is "Why do mirrors reverse
right-left, but not up-down?", which in fact is the name of a short story
somewhere if I'm not mistaken.

However, the usual answer to this problem (pointing out that flipping the image
in the verticle axis rather than the horizontal axis is a convention due to
gravity bias) I find unconvincing.  So here is a more convincing (to me)
explaination.

Imagine yourself facing north just in front of a south-facing mirror.  Point
east.  You image points east.  Point west.  Your image points west.  Point up.
Your image points up.  Point down.  Your image points down.  Point north.  Your
image, tricky fellow, points *south*.  And when you point south, your image
points north.

Therefore, it seems simple to conclude that mirrors reverse front-back, not
left-right, nor up-down.

Note that by a front-back image reversal, mirrors reverse *handedness*, which
is conventionally associated with left-right.  This, coupled with the above
mentioned gravity bias (things are normally assumed to be symetrical around
vertical axes), leads to the common (but mistaken) notion that mirrors reverse
right-left.
-- 
Wayne Throop at Data General, RTP, NC
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