# Mirrors

Good question posed by warmbrain:

Why do mirrors reflect left-to-right, but not up-and-down? I mean, they’re mirrors right? How do they know?

Hmmm…. is it due to the horizontal orientation of our eyes? If our eyeballs were stacked top-to-bottom on our faces rather than left-right, would mirrors reverse the up and down?

Music: Altai Hangai :: Praise song for Bogd Khan Mountain

## 14 Replies to “Mirrors”

1. Vincent Zee says:

just imagine the trajectory of the photons comming from the object to the mirror.

2. i says:

I read an explanation of that once, after thinking about it for hours, and even after the explanation the whole situation still drives me crazy.

3. baald says:

ok, try this: imagine a painting that is still wet. put a piece of paper on it and get a transfer of that painting. it will be “reversed” left to right. but not up-and-down.

in actualitly, nothing is reversing, so there shouldn’t be any expectation of a vertical flip.

same thing applies with a tranparency. turn it around, and it’s mirror-imaged. better yet (a better demo of my example above): get two transparencies that are identical. stack them on top of each other so they look like one. now pull hte top one straight off the bottom one, but keep their surfaces parallel, till they are about 18″ apart. now stick your head in between them. the bottom one looks the same, but turn your head around, and the other one will now be mirror-imaged.

easier to picture than to explain….

4. the issue is where we have symmetry… baald has it right. try lying on your side and looking in the mirror. is it still reflecting left to right and not up and down?

5. Lars says:

The mirror is just reflecting the same in all directions.

What makes it tricky is that the brain has only the eyes to distinguish left from right, and the meaning of the word is furthermore only relative to ones position. For up and down we have physics as reference (ie. gravity), so no confusion can arise.

6. The left-right thing is just the way we perceive direction.

But the thing that really got my attention was Altai Hangai. Just last night I listened to them and decided to order the same album. What a coincidence!

7. Lars says:

And to go a bit off tangent: the preview in Apple’s iChat AV is side-reversed like a mirror, and many people apparently considered that a bug. But it’s not – Apple figured (correctly) that most people know by practice how to read a mirror image, but would get hopelessly confused with a ‘correct’ image.

8. I get it. I guess I was thinking that the reversal was like lens reversal — which would be upside down and backwards. But no, there is no reversal, we’re just seeing the image from the wrong way ’round. Still, like an optical illusion, your mind plays tricks on you if you think about it too much.

9. to simulate photons and a mirror:
fly paper=mirror
paint=photons

step 1. paint left side of face blue, right side red
step 2. put fly paper on face
step 3. remove fly paper
step 4. if your eyes were not removed from their sockets, look at the fly paper, notice make up. blue on left. red on right.
step 5. go to hospital

10. Tyler says:

more to it than that, and less…

A mirror really is reversing something, the Z axis going into and out of the mirror. Reversing a single axis turns a “left handed” object into a “right handed” object, and vice versa. This means that if you started with your watch on your left hand, your mirror image has it on its right hand. Two totally asymmetrical objects don’t look the same at all, they just look like two totally different objects.

For instance, consider a helix of DNA. If you’re holding it, it curves up and to the right. Flip it over, and it’s still up and to the right. Rotate it however you like, and it’s always up and to the right. The mirror image is always up and to the left, no amount of rotation will make one look like the other.

In the case of people though, there is bilateral symmetry. An object with a plane of symmetry has no “right handed” and “left handed” version, they are the same thing, consequently a handedness inversion really just looks like a 180 degree rotation through the plane of symmetry, the two operations are equivalent.

Therefore, people see this as a 180 degree rotation, even though people aren’t really fully symmetric, just wear a watch, or get a scar and you’ll see that your mirror image isn’t any sort of rotation of you, but if you’re willing to overlook the difference (and humans are hard wired to do so), it looks like a 180 degree rotation.

11. Now that that’s settled (IÂ guess), maybe someone can explain whether a closed container with birds in it is or is not lighter if you go out and bang on the container to make the birds fly….

12. Bill – Of course such a box would be lighter when the birds are flying inside it. Why wouldn’t it be?

13. The total mass inside the truck is not decreased by making them fly; to make it clearer, the column of (air+bird flesh) over each square inch of the floor of the truck weighs exactly the same, whether the birds are in motion or not…. Alternately, the fact that, between the bird and the floor of the truck, the mass of bird flesh is supported by air rather than a crate should mak no difference: weight just doesn’t disappear, of course.

On the other hand — remove the birds from inside the truck, and have them perch on top of it, and the problem seems to vanish: surely the weight of a truck is not increased by a bird (or a plane) flying over it a thousand feet up! (Although again, that column of space above the floor of the truck does include some bird flesh or plane metal; we allow for the air in everything — scales are de facto calibrated for it, so to speak.)

It is a mystery, right up there with mirror reflections.

Martin Gardner devotes an entire chapter of one of his books, by the way, to the mirror problem: it makes for very dense reading, and didn’t convince me one bit, I got the feeling he was pulling the feathers over my eyes.

14. > It is a mystery, right up there with mirror reflections.

Bill, this problem is more fascinating than I thought. But it doesn’t sound too hard to test empirically. What happens when the experiment is actually tried?

I’m thinking now of the reverse problem, and of the relation of buoyancy to all of this – a scuba diver suspended in water weighs less than the water he displaces, so the volume of water surrounding him weighs less than it did before he entered it. Now, let’s assume he has neutral buoyancy due to wetsuit and weight belt and b.c.. What if you take away his b.c. and weight belt so he has to swim to stay in place. Does that change things due to pressure on the space exerted by swimming?

Wow, this one is tricky.