A mirror is so smooth that light bounces off at the exact same angle it arrived, like a perfect bank shot, so you see a tidy copy of whatever's in front of it.
Most people think a mirror swaps your left and right sides. In fact it flips nothing left to right; it reverses front to back, pushing your reflection through itself nose-first, and you read that as a left-right swap.
What's actually happening
Every surface reflects light — that is the only reason you can see this page. The difference with a mirror is order. A wall is microscopically rough, so parallel rays hitting it bounce off in scrambled directions and the picture they carried is destroyed. A mirror is smooth at the scale of light itself, so every ray obeys the same clean rule: it leaves at exactly the angle it arrived. The picture survives the bounce intact.
Your brain, however, has no idea the bounce happened. It assumes light travels in straight lines, so it traces each incoming ray backwards, straight through the glass, to where the rays appear to come from. They converge at a point exactly as far behind the mirror as the object is in front. That phantom, the virtual image, is why a mirror seems like a window into a duplicate room.
And the famous puzzle: why does a mirror swap left and right but not up and down? It doesn't. A mirror flips front-to-back, nothing else. Your reflection is not you rotated — it is you pushed through yourself, nose-first. We read that inversion as "left-right swapped" because we imagine turning around to become the reflection. The mirror never turned anything.
A mirror keeps every angle honest, light leaves exactly as it arrived, and places a virtual image as far behind the glass as you stand in front.
- 1Stand a small mirror upright on a table and place a coin 10 cm in front of it.
- 2Look into the mirror at a shallow angle and slide a second coin around behind the mirror until it appears to sit exactly where the reflection is.
- 3Measure both distances to the glass: they match. The image is a precise geometric ghost, the same distance behind as the coin is in front.
Common questions
A mirror is smooth at the scale of light, so every ray leaves at exactly the angle it arrived. A rough wall scrambles those rays in all directions and destroys the picture; a mirror keeps it intact.
Your brain assumes light travels in straight lines and traces the rays back through the glass to a virtual image. That image sits exactly as far behind the mirror as the object is in front.
The lettering is reversed so that the front-to-back flip in a driver's rear-view mirror un-reverses it, letting the word read correctly to the car ahead.