Many people believe that mirrors reverse left and right, but not up and down. Actually, they don’t do either. To find out what’s really happening, try this series of experiments with a “buddy”—your own reflection.
Stand in front of the mirror. Point a finger upward. Which way does your reflection point? Point downward (see photos below). Which way does your reflection point?
Imagine west is to your left and east is to your right. Write “West” and draw an arrow pointing left on one piece of paper. Write “East” and draw an arrow pointing right on another piece of paper. Tape the signs on their appropriate sides, somewhere you can see them, but where they don’t have a reflection in the mirror.
Look in the mirror and point to the west (left). Which direction does your reflection point? Point to the east (right) (see photos below). Which way does your reflection point?
In your imagined orientation, north is the direction into the mirror. Write “North,” along with an upward-pointing arrow, on a piece of paper. Place it on the floor pointing north, or towards the mirror, preferably where you can’t see it in the mirror. Then make a similar sign for south, writing “South,” along with a downward-pointing arrow, on another piece of paper. Place it on the floor pointing south, or away from the mirror, but where you can’t see it in the mirror.
Look in the mirror again and point to the north. Which way does your reflection point? Point south (see photos below). Which way does your reflection point?
Finally, put the opaque paper behind the transparency so that the “F” faces you and looks normal, like the letter “F” (see photo below). Make a prediction: What will the reflection look like when you remove the paper? Now try it!
When you point up, your reflected self also points up. When you point down, your reflection also points down. Similarly, when you point west, your reflection points west. And when you point east, your reflection also points east. So much for the (false) idea that mirrors reverse left and right.
The actual reversal that’s taking place becomes clear when you point north, into the mirror—and your reflection points south, out of the mirror. Similarly, if you point south, the reflection points north.
You’ve just discovered the truth about mirrors: They actually reverse in and out. Another way to think about it is that mirrors reverse front and back.
Text looks reversed to us in mirrors because of this back-to-front reversal. In a reflection, the backside of the text is facing you, instead of the usual front side. Since we usually print on opaque media, we don’t have much experience seeing text from the back—with one exception: You might have seen writing from the back when looking out the window of a glass-fronted shop. Text seen this way—from behind and through the glass—looks reversed in exactly the same way that writing in a mirror looks reversed (see photo below). Your experiment with the transparency makes the situation perfectly, ahem, clear.
You can try doing this mirror experiment without a mirror—working instead with two people standing face to face. The may agree on whether they’re pointing east and west, but do they agree on whether they’re pointing left or right?