Mirror mirror, right and left, up and down
You are getting ready for the New Year’s party and cannot help to use a mirror to check that all is spot on. The tie is straight, the hair is tamed, the shoes are polished but wait… right is wrong, or rather right is left and left is right… but up is still up and down is down. Why, you ask, do mirrors reverse right and left but not up and down? Well, the answer is that they do not do either of them. They reverse front to back…
The image that you see in front of you has not been swapped, but inverted along the axis of the mirror. So the answer to this question can be understood with looking at how light gets reflected. If we consider a light source, its rays will bounce off various parts of your body, they will reflect off the mirror and will be caught by your eyes; plus we know consider that for all intents and purposes light travels in a straight line. And so, a mirror (not a fun fair mirror by the way) will simply reflect what is in front of it: the light bouncing of your right hand will hit the mirror straight on and then will bounce into your eye. What you will perceive is that you see your right hand in the place of the left one. And notice that it is a matter of perception… Now, try the following: position yourself looking North and place the mirror in front of you. Now point at something East with your right hand, you will see that the hand in the mirror will also point East; the same happens if you point West with your left hand. So the directions are fine: East is East and West is West. But look at your nose, it points North, right? What about the nose in the image? Well, it points South! The image is reverted front to back.
Richard Feynman provides this explanation to the BBC TV Series “Fun to Imagine” in 1983.
Now, you have something to think about next time you are getting ready in front of the mirror.