Science Questions

Why are number plates reversed in my rear-view mirror?

Mon, 14th Apr 2014

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Question

Ocsi Veres asked:

How come the image of roadsigns, number plates etc appear the wrong way around in my rear view mirror, yet the countryside I have just passed trough appears the right way around?

Answer

Ramsey - Its a great and potentially tricky question. So certainly, everything is Car wing mirrorthe wrong way around. So, trees and buildings have fantastic symmetry so its hard to glance at tree and determine if you're seeing it the right way around or its reflection. Text we're obviously extremely good at looking at and recognising instantly if its the wrong way around or not.

The big reason is basically, the difference between a reflection and rotational symmetry. So, if I stood in front of my identical twin brother, who doesnt exist, but for the purpose of this he does, if I stood in front of an identical twin brother of me and I held out my right hand to shake my right hand, he would go through the same motion, hold out his right hand. Our arms would cross in front of us and we would shake hands. And to all intents and purposes, wed done the same thing. We both used our right hands, but if I tried to do this with my reflection in a mirror, and Id put my right hand out, my reflection in the mirror would appear to put out its left hand and we couldnt possibly shake hands. Wed be holding the wrong hands out. That is basically the crux of the difference. When the text that you look at in a mirror is presented back to you, you're sort of looking at the back of it. You're looking at its reflection, not its 180 degree rotation in space which is what you really need in order to read it properly. So, its the big difference between reflection and a rotation. Mirrors reflect but we need that text to be rotated rather than reflected to read it properly.

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