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
Why Do Mirrors Flip Objects Horizontally But Not Vertically?
