Why do mirrors flip left and right but not up and down?
Does a mirror flip left and right? Guess first, then try it with real coordinates and see what a mirror changes and what it doesn't.
- 1A mirror reverses front and back, nothing else6 min
- 2The image is behind the glass, as far as you are in front5 min
- 3Left and right swap only in our imagination6 min
- 4Two reversals cancel, and show you as others see you5 min
Sound like you?
The ideas people most often get wrong on the way, in their own words. Each step starts from one of them.
The short answerRather read it than work it out?Show it (it gives the steps away)
A mirror is a flat plane, and it swaps only the direction pointing into it. A pen 40 cm to your right, 100 cm up and 60 cm in front has an image 40 cm right, 100 cm up and 60 cm behind. Sideways and up are unchanged; only the distance from the glass changes sign.
The reflection is a virtual image that seems to sit behind the mirror at the same distance as you are in front of it. At 2 m from the glass, your image is 4 m from you. Walk at the mirror at 1 m per second and you close the gap at 2 m per second.
We read a reflection as another person facing us. To face us, a real person would have turned round about an upright axis, and that swaps left and right hands. The mirror hasn't turned anything: it reversed front to back. Closing one eye, or using a camera, changes nothing about this.
One mirror reverses you front to back, so you see yourself reversed. Two mirrors set at a right angle reverse twice and the reversals cancel, so you see yourself as other people do. That is also why your own photos can look wrong: they are the right way round, and you are used to the reversed version.
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