That’s a good question! If you hold a sign up to a mirror, the words appear backwards, but not upside-down. Why? Dianna the Physics Girl explains.
Objects in mirrors aren’t flipped on the X-axis (horizontal) or the Y-axis (vertical) but the Z-axis (depth). Objects only appear to be flipped horizontally because our minds are doing that. Our bodies are symmetrical along the Y-axis. So when we look at ourselves in the mirror, we don’t perceive that we are being flipped along the Z-axis.
-via The Presurfer
When I was 7 and asked my father the same question, he said, "For the same reason that there's a North Pole and a South Pole, but not an East Pole and a West Pole."
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Think of the image in a mirror as a rubber mould of the world this side - turned inside out. Top is still at the top, bottom at the bottom. Left is still on the left and right on the right. Near things are still near - far things are still far.
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