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On the Today programme this morning they had a science Q&A. They did the Qs just before I left for work, so unfortunately I missed the As.

One of the Qs was something along the lines of "Why do mirrors flip left and right, and not up and down?". Having heard the question before, I do know the answer, but still find it fascinating whenever it comes up, as it's one of those questions where the answer is completely obvious and entirely non-obvious at the same time (one of those kick yourselves when you find out the answer for not spotting it yourself type things).

The answer is, obviously (just to rub it in for those who don't know the answer already and like me didn't spotted it for themselves), that mirrors in fact flip front and back and not left and right. When you look at your reflection, your reflected left is still on your left and your reflected right is still on your right. The idea of these being flipped comes from the fact that your reflection is facing the other way and left/right depend on the direction you are facing whereas up and down don't.

I find it fascinating that the vast majority of people (including myself) pick up the misconception that mirrors flip left and right. Why is this? Is it due to discussing other people's left and right and considering our reflections as a separate entity? Do young children who are only just learning about left and right have this misconception?


( 5 comments — Leave a comment )
7th Mar, 2008 11:52 (UTC)
It all depends on how you define left and right.

If you define left and right so that both I and the image have their own, or in terms of chirality like a chemist, then if I move my right hand, the image moves its left hand. A right hand glove in real life looks like a left hand glove in the mirror.

If you define left and right as my right and left, as the position of an object and not an integral property of the object, then if I move my right hand, the image moves the hand that is also on my right. It is my right but its left.
7th Mar, 2008 12:16 (UTC)
Yes, my left hand will be reflected as my reflection's right hand. However, it's still not left and right that has been switched. They've stayed constant while the direction I/my reflection is facing has been flipped.

Edited at 2008-03-07 12:17 (UTC)
7th Mar, 2008 12:45 (UTC)
But if you count left and right as a local co-ordinate system with its origin in the middle of the person or image-person that it's local to, then it looks flipped. Also, for things like gloves, leftness and rightness are a topologically non-changeable property, and the image has the opposite leftness/rightness to the object.
7th Mar, 2008 13:21 (UTC)
I think that's the point I was trying to make wasn't it?

That the reflection will have it's left/right labelled the opposite way round to the image (assuming you don't flip the local coordinates system with the object), because it has been flipped forwards/backwards and is now facing the other direction. This makes it look as if left/right have been flipped when they actually haven't.
7th Mar, 2008 13:25 (UTC)
I don't know if it was...I think I am thinking about it in a chemist like way and you're doing it like a mathematician.
( 5 comments — Leave a comment )

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