You may have always wondered why a mirror reverses left and right, but not up and down. Technically, it isn't correct to say that a mirror reverses left and right. A mirror reverses along lines perpendicular to its surface:
- If you were to put a mirror on the floor and stand on it, the glass is perpendicular to the up-down coordinate axis. In this case, the mirror reverses top and bottom (not left to right, or front to back).
- If you place a mirror on a wall and stand with your side to the mirror, your left-right axis is perpendicular to the mirror, and so the mirror would reverse left and right.
- If you place a mirror on a wall and face the mirror, your front to back axis is perpendicular to the mirror, so the mirror reverses top to bottom while keeping left to right and top to bottom as they are. Because the image of your left hand is opposite to where it would be if you were to step behind the glass, people say that the mirror has reversed right and left, but it hasn't really.
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