A very smooth wall
How Do Mirrors Work?
Every surface bounces light. A mirror is just flat enough to keep the picture intact.
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Step 1 of 5
You only ever see bounced light.
Objects that do not glow are visible because light lands on them and bounces into your eye. That is true of a wall, a face, and a mirror alike.
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Step 2 of 5
A rough surface scatters the bounce.
Zoom in on paper and it is a mountain range of fibres. Each one throws light off at a different angle, scrambling the picture completely.
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Step 3 of 5
A mirror is flat down to the atom.
Flat down to the atom. That is the whole difference between this and a wall. A thin layer of aluminium behind glass is smooth to a fraction of a wavelength. Every ray bounces off at a matching angle, so the picture survives the trip.
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Step 4 of 5
The angle in equals the angle out.
Light bounces off a mirror like a ball off a wall — same angle, other side. That single rule lets you predict exactly where every reflection lands.
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Step 5 of 5
Your reflection is not actually reversed.
A mirror does not swap left and right — it swaps front and back. You raise your right hand and the reflection raises the hand on the same side of the room.
Mirrors are very slightly green. Glass absorbs a little red, which is why a mirror-lined corridor looks greener the further you look.
The short version
A mirror works because it is smooth enough to bounce every light ray without scrambling the picture.
Try it yourself
Look at your reflection in a spoon's back, then its bowl. Curving the same shiny surface changes everything.