Do mirrors really reverse left and right?

Raise your right hand in front of a mirror, and the you on the other side appears to raise their left. Hold up a book, and the title on its cover comes out backward and hard to read. Mirrors reverse left and right: that is what most people believe.

But stop and think for a moment. As far as the mirror is concerned, left–right and up–down are both simply directions running along its surface. And yet it never shows your head at the bottom and your feet at the top. Why would a mirror play favorites with left and right? This innocent question has sparked debate among scientists and philosophers for a long time.

What a mirror flips is front and back

Stand in front of a mirror and point to your right with your right hand. The hand in the mirror points to the same side, the right. Point up, and the finger in the mirror points up; point down, and it points down. In fact, the mirror shows both left–right and up–down exactly as they are.

So what does change? Thrust a finger forward toward the mirror, and the finger in the mirror comes pointing back toward you. What a mirror reverses is the direction perpendicular to its surface: front and back.

What happens when front and back are reversed? A glove makes it easy to see. Turn a right-hand rubber glove inside out, and it becomes a left-hand glove. The image in a mirror is turned inside out front-to-back in just the same way. That is why the reflection of a right hand has the shape of a left hand.

The left–right swap happens in your head

Then why do we feel that left and right are reversed? Because we see the image in the mirror as another person standing there facing us.

When we face someone, we think of them as having turned around to look our way. Turning the body half a circle around a vertical axis keeps up and down in place and swaps only left and right. The moment we treat the reflection as “me, turned around,” the relationship is set: the reflection’s right hand corresponds to our own left.

But the person in the mirror hasn’t actually turned around. They have only been reversed front to back. We are thought to choose the “turned around a vertical axis” interpretation because our bodies are nearly symmetrical from left to right, but completely different from top to bottom.

Try lying down on your side in front of a mirror. If you lie with your head to the right and your feet to the left, the you in the mirror also has their head on the right and feet on the left. The hand on top is also on top in the mirror. Even so, the person in the mirror will seem to be “raising the opposite hand.” That shows that what feels reversed is not left and right in the room, but left and right measured by the body.

Backward writing works the same way

When a book’s title comes out backward in a mirror, the mirror hasn’t flipped the letters on its own initiative either. It happens because, to show the cover to the mirror, we turned the book around in our hands to face the glass. Turn it around a vertical axis, and the letters appear reversed left to right; turn it around a horizontal axis, and they appear upside down.

For proof, write some words on a clear sheet and hold it up in front of the mirror, still facing the way you can read it. The writing in the mirror reads perfectly normally. You haven’t turned the sheet around, so the letters are not reversed.

On the other hand, with two mirrors you can meet a “non-reversed you.” Join two mirrors at a right angle and look into the seam where they meet. Raise your right hand, and the person in the mirror raises their own right hand. The two reflections cancel out the front-to-back reversal, showing you as other people see you. It may look oddly different from your familiar mirror face, and feel a little unsettling.

This property is put to use in daily life, too. Ambulances in Europe, North America and elsewhere sometimes have the word “AMBULANCE” written in reverse across the front. That way, drivers ahead can read it correctly when they see it in their rearview mirror.