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View Full Version : Why does a mirror only flip you horrizontaly and not vertically?

MetropoChris
11-30-2000, 03:56 PM
I know this sounds like a dumb question but I cant seem to put the answer into words. It's silly to think the mirror would filp you upside down, but tough to explain why not at the same time. If you began to rotate a mirror 360 degrees your image would not change. But if you layed down in front of a mirror it would flip you vertacally but NOT horrizontally. How does it know to only flip left to right and not up to down. Again, it seems obvious, but I cant put it into words...

gigi
11-30-2000, 04:07 PM
It doesn't flip you horizontally, it reflects back what's in front of it. Your left side is on the left side of the image, etc. It shows you as others see you, but it doesn't flip your image.

c_goat
11-30-2000, 04:07 PM
A curved mirror will flip you both ways (like a spoon), or maybe just upside down. It's the way the light bounces off the mirror, and the way your brain assumes it came to you in a straight line. I can't explain without drawing a diagram. Maybe someone else can.

Arjuna34
11-30-2000, 04:10 PM
Because your two eyes are arranged horizontally, not vertically. It really doesn't flip anything.

Arjuna34

gigi
11-30-2000, 04:12 PM
Oopsy--I was picturing that all wrong--I should know to leave it to the master:

http://www.straightdope.com/classics/a2_082.html

City Gent
11-30-2000, 04:20 PM
Arjuna34, it really does "flip" you, but there's some subtlety to it. Richard Feynman explained this one very succinctly. I can't find the source right now, so I'll try to summarize it as best I can.

What the mirror is really doing, geometrically, is reversing you front-to-back, because your front and the mirror's surface have opposite orientations. We don't usually think of it this way, because it's hard to visualize (try picturing your body morphing so that the front turns into the back and vice versa - it's difficult.) The effect of the front-to-back reversal is that your left hand now corresponds to the image's "right hand", but your top still corresponds to the image's top.

Said differently, the symmetry plane that the mirror reverses you about is a vertical plane that runs through both your shoulders, not a vertical plane that splits your face in half, like the left-to-right hand reversal suggests sometimes.

malden
11-30-2000, 04:31 PM
For a mirror that shows us the way others see us, wouldn't two flat mirrors perpendicular to each other do this? The orientation would be like facing a corner of a room and mounting mirrors on both walls. Move your right hand, and your right hand's reflection will move (not the reflection's left hand, as would happen with a flat mirror.)

I think I have observed this phenomenon before, but I don't have any mirrors handy right now.

Harmonious Discord
11-30-2000, 05:11 PM
malden
I know that someone had produced a mirror that does that. The image you saw in the mirror would raise it's right hand when you raised your right hand. I saw this quit a while ago.

Harmonious Discord
11-30-2000, 05:28 PM
Here's a link to a site that shows you a picture of the unit. Weird sales pitch included on site.
http://www.truemirror.com/

malden
11-30-2000, 06:55 PM
...Then what about Cecil's reference to "a complex combination of concave and convex surfaces" that produces a true image? (See the archive column in the link given in an earlier post.)

The place where I saw this phenomenon was the restroom of some restaurant. Two perpendicular walls had mirrors on them. It is a really interesting effect, even though there is a little line down the center of your reflection.

Harmonious Discord
11-30-2000, 09:11 PM
It looks like the mirror actualy involves two reflective surfaces at 90 degrees to each other, that reverse the image.
Drop them an email. They may give you a response.It worked for the hole in the can of shoe polish question a year ago. The following link summerizes this quest.
http://www.straightdope.com/mailbag/mkiwi.html

dtilque
12-01-2000, 12:37 AM
Originally posted by City Gent
What the mirror is really doing, geometrically, is reversing you front-to-back, because your front and the mirror's surface have opposite orientations.
This is correct, mirrors do flip you front to back, i.e. perpendicular to the plane of the mirror.

The effect of the front-to-back reversal is that your left hand now corresponds to the image's "right hand", but your top still corresponds to the image's top.

No, the reason that you think your left hand is the image's right hand is that you are interpreting the image as if you were facing another person. You aren't. The reflection of your left hand is on the left part of the mirror, which is expected if it doesn't flip left-right.

Writing seen in a mirror is flipped left-right, but that's because you have to rotate the sheet or book to face it into the mirror. The rotation is what does the flipping, not the mirror.

keeper0
12-01-2000, 01:27 AM
I know that someone had produced a mirror that does that. The image you saw in the mirror would raise it's right
hand when you raised your right hand. I saw this quit a while ago.

I had the damnedest time combing my hair in one of those mirrors. My hands kept moving in the wrong direction, and when I was done the hair was parted on the wrong side.

sdimbert
12-01-2000, 09:47 AM
This has been answered, but I need to pipe in with some "reflective" (heh) nostalgia... This was one of (if not the) first questions I posted here, over a thousand posts ago. Check it out here (http://boards.straightdope.com/sdmb/showthread.php?threadid=10455).

LateComer
12-01-2000, 01:21 PM
Originally posted by dtilque
Writing seen in a mirror is flipped left-right, but that's because you have to rotate the sheet or book to face it into the mirror. The rotation is what does the flipping, not the mirror.
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This idea has helped me to understand this concept.

Writ on a piece of paper that you can see through (tracing paper maybe) and turn it to face the mirror. Look at the writing on the paper and in the mirror. They are both aligned the same way.