Physics
Chennai
Coimbatore

Why doesn't lateral inversion apply vertically?

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February 04, 2013 3:01PM

The simplest answer to this apparent contradiction: When you

look in the mirror, your right ear appears on the right-hand side

of the mirror, and your left ear on the left. So surely you were

not expecting the top of your head to appear at the bottom of the

mirror and your neck at the top? Of course you weren't, by the same

argument.

A deeper answer (but still basically very simple): Any object

that does not have a plane of symmetry is said to be chiral (in

chemistry the term and the concept is often applied to molecules,

and is especially important in biochemistry and pharmacology). When

you view a chiral object in a mirror, the image that you see has

the chirality reversed. Thus, for example, you look like someone

with hair parted on the left, whereas yours is actually parted on

the right. This is called "lateral inversion". It is an unfortunate

and misleading term, as it seems to suggest a sideways inversion,

i.e. that left and right are interchanged in the image that we see,

which certainly does not happen in the sense of shifting from one

side to the other (see above). The reversal of chirality is the

complete story - there's really nothing more to be said about it.

No question about the image being turned upside-down should

arise.

Comment: Here's a fairly simple explanation, but it's "deep" too

and it's correct.

1) Lateral inversion occurs with plane (flat) mirrors.

2) The mirror only SEEMS to reverse right and left. So that gets

rid of the problem in the question, but it raises the question of

why it SEEMS to reverse

right and left.

3) The mirror actually just reverses points on the object in the

direction that's

perpendicular to the mirror's surface. That's ALL it does.

4) However, the mirror image is often perceived ("seen") as

being left-right reversed, with front-back being "seen" as

unchanged.

That's the bit that's hard to explain. It's to do with the

psychology of visual perception. If you look at yourself in a

mirror it is really hard to try to "see" yourself as reversed front

to back.


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