[Rich]

Rich Morris asked the Naked Scientists:

Can you explain why when you look in a mirror, you are flipped left to right, but not up to down (...unless you lie down)?

TIA
Rich Morris, Madison Wisconsin


What do you think?

[lyner]

Stand in front of a mirror and windmill you right arm. Head,right,feet, left,head right,feet,left etc.
The image would appear to be windmilling in the other sense - head,(his) left,feet,(his) right etc.
This would happen even if you were laying down - You would still see the image of your head the same end as  your head. So it's not up / down, or left / right that have changed  it's the rotation that's changed.
What has really flipped is the front and back of the person in the mirror. If the image were a true version of you (transferred into the space behind the mirror), you would be looking at your back and you would be facing away from yourself. Your image has actually flipped front to back - not side to side or top to bottom, so you are face to face.
Look at a large screw in the mirror and compare the original and its image, right next to it - one has a right hand thread and one has a left hand thread, whichever way up you hold it.

[Rich]

That's a nice reply.  Thanks much. 

I've been asking this question for several years.  In fact a friend uses it on people defending their thesises... (or is it thesii?)

It's such a simple question, and we all know the behavior.  Yet, it is difficult to explain to someone at a cocktail party in simple, yet clear terms.  I'm wanting (hoping) for the one sentence, layman's explanation with a one sentence followup example.

[science_guy]

one of the best ways to describe a phenomon is by a diagram (sometimes)

[diagram=346_0]

if you look past the horrible drawing, you'll see the path of light for the two light sources, the red and the yellow, heading exactly as they should for the path.  Notice the direction, when the lights reach the right side, and how they are relative to the sides of the face.

[diagram=348_0]

the sources of light, which have not changed, are now shining into a mirror.  The face, which is now looking at the mirror, is what has actually been reversed, since the face has been turned around. notice that the yellow light is now shining into the face's right, rather than his left.


pretty bad drawings of faces though  []

[lyner]

But you would see both lights with both eyes, wouldn't you?

[LeeE]

Actually, you aren't flipped left-to-right at all, and that's why you're not flipped upside down either.

The illusion of the reflection being flipped left-to-right comes from an internal assumption that the image has been rotated through 180 deg around a vertical axis running through the mirror.  It hasn't, of course, it's only been reflected, which is why it also isn't rotated top-to-bottom.

[science_guy]

but it is not the image that has been rotated, it is what is recieving the image that has rotated.

[lyner]

I don't spin round when I look in a mirror?