# Mirror and reflection

ok, i heard a question asked on a tv show last night but i missed the answer. it feels like this should be an easy one but i’ve gone locked the brain gears.

why is it that text reflected in a mirror is seen in reverse side to side but not inverted top and bottom?

cheers all,
my poor weary brain thanks you in advance

Whe nwe say that text is inverted, what we really mean is that it’s bounced back in the same way that it’s presented.

I stand in front of the mirror, holding an open book so that two pages face the mirror. Then from my perspective the text on those pages is going right to left, which is exactly what the mirror shows me.

thanx, just as i thought. i tried to make an easy answer hard and resulted in an acute case of stimied noodle

I once heard that answer to this is that the writing isn’t reversed, you’re just standing on the wrong side of the mirror.

1. Take a clear sheet of plastic or glass and write something on it with a dry erase marker (or whatever). Hold the sheet up in front of you so you can read it. Now walk in front of a mirror. The reflection of the sheet will be readable exactly the same as the sheet in front of you. There is no reversal at all.

2. Take a book or something else with a page of writing and stand in front of a mirror. Hold it up in front of you so you can read it. Now tip the top of the book towards the mirror until it is parallel with the ground; you can still read it. Now look at the reflection in the mirror. The writing is now reversed from top to bottom but not left to right, the opposite of what’s described in the OP.

Many people have tried to explain this to me but I have never understood it. It seems that if your left and right eyes change place then your feet and head should as well.

Basically you are rotating around one axis but not the other when looking into a mirror. There is nothing intuitively obvious about why this should be the case.

Would things be different if our eyes were aligned along a vertical axis instead of a horizontal one? :dubious: That’s the only way I can make it work in my head.

Ordinary mirrors reflect things front to back, not left to right. For instance, my bathroom mirror is on my west wall, so when I look in it, I’m looking west. But my reflection is looking east. On the other hand, if I raise my south arm, the reflection also raises its south arm.

The only reason we think of mirrors as reversing left and right is that humans are, for the most part, bilaterally symmetric. If I stand in front of a mirror, I see what looks like another human standing on the other side. When I raise my south arm, that’s my left arm. But if that were another person on the other side of the mirror, instead of my reflection, that would be that person’s right arm.

It is a frame of reference question. And it also stems from the fact that we are so used to mirrors being placed on walls.

If you place the mirror on the ceiling then your image will indeed appear upside down.
If you are looking directly into a mirror and you are facing north, then your image will face south.
If you stand side on to a mirror then you will be upright and facing the same way as your image but you will be inverted left to right.

Hang on a moment, aren’t you always inverted left to right? If you wear a watch on your left hand, doesn’t your image wear the watch on its right hand?
The answer is that we define up/down and north/south with an absolute frame of reference – consistent with the world in which we exist (ie, the object).
But we define left/right with respect to ourselves when we are talking about ourselves. However we use the image’s frame of reference when talking about left/right on the image. In the case of standing side on to a mirror, these two frames of reference are the same. But in any other orientation the two frames of reference are different.

It isn’t - I just wrote ABC on a piece of paper and looked at it in a mirror; the A was the rightmost character and the C was the leftmost on the image of the paper in the mirror - exactly the same as the actual piece of paper in my hand.

Well, here’s a broken mirror in action.

Serves the guy right for mounting it backwards.

As Chronos says, what a mirror is really doing is reversing front to back. There’s no rotation. So imagine slicing up the world into lots and lots of very thin slabs, all parallel to the mirror. Identical slabs are on the “other side” of the mirror plane, but in reversed order.

(And actually, I think you can regard mirror reflection as a 180-degree rotation, if you allow your space to be four-dimensional. But I don’t think that’s helpful here.)