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.
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.
- 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.