Science: Why do mirrors flip horizontally but not vertically?

Physics Girl. When you look in a mirror, you’re looking at a horizontally backwards version of yourself, right? Wrong. It’s more complicated than that.

For example, when you hold up a sign saying ”Food” in the mirror”, it flips reads a backwards ”dooF”, but the letters are still the right way up. And why, when you raise your right hand, your mirror-self raise its left hand, but it still moves it up rather than down. How does a mirror know the x direction from the y direction?

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