General Knowledge
Why does a mirror swap left and right but not up and down?
- It swaps front and back, and you do the rest
- Light bounces sideways
- Your eyes are side by side
The answer is It swaps front and back, and you do the rest. It reverses front to back; you turn yourself around to compare.
It does not swap left and right. A mirror reverses front and back - the direction along its own axis - and everything else follows from what you do next. Your reflection's nose points towards you while yours points away; nothing has been swapped sideways at all.
The confusion comes from how we describe the person in the mirror. To say they have raised their left hand, you imagine turning yourself around to face the way they face - and it is that imagined rotation, about a vertical axis, that introduces the swap. Rotate about a horizontal axis instead, by imagining a handstand, and you would say the mirror flips top and bottom.
Lie on your side facing a mirror and the reversal appears to run head-to-toe, which is the giveaway: the mirror has not changed, only your idea of which way is up. Text is a clean demonstration - hold writing to a mirror and it reverses, but hold it up to the light and read it through the page and it is fine, because you turned it the other way.