Roll a ball hard at a smooth wall and it doesn't stop — it bounces off and comes racing back at you. Light does the very same thing. When a ray of light meets a smooth, shiny surface, it doesn't sink in and it doesn't stop. It bounces cleanly off and carries on its way. That neat, tidy bounce is called reflection.
This is the secret of the mirror. A mirror is just a flat sheet of glass with shiny metal behind it — there is no picture of your face painted on it. Yet every morning you look in and there you are, staring back. Your face appears because the mirror is so smooth and shiny that it bounces all the light off in an orderly way, keeping the picture together. Still water, polished metal, and a clean shop window can pull off the same trick.
Here is the one rule the bounce always obeys. Watch a single ray of light strike the mirror. It comes in from one side and leaves on the other side, like a ball off a wall. The dashed line pointing straight up from the bounce point is our helper line — it just marks "straight up" so we can compare the two sides.
Tilt the incoming ray with the slider and watch what happens: the ray bounces off at the same angle it arrived. Lean it in more and it leaves more; lean it in less and it leaves less. The angle going in and the angle coming out are always a perfect matching pair — never one bigger than the other.
Here is a puzzle. Every surface bounces some light back — that is the only reason we can see anything at all. So why do you see a clear picture of yourself in a mirror, but only a dull grey in a brick wall? Both bounce light. The difference is how smooth they are.
A mirror is smooth as glass. Every tiny part of it faces the same way, so all the rays bounce off neatly, side by side, and the picture reaches your eye whole. A wall is covered in tiny bumps and grit, and each bump faces a slightly different way. So the wall flings the rays off every which way — the picture is scattered into a jumble before it can reach you. The light is still bouncing; it's just been shuffled into a mess.
This is why you can see your face in a still lake but not on the pavement beside it. The lake's surface is smooth, so the picture survives. Now blow a breeze across the water: the ripples make the surface bumpy, and your reflection breaks into a wobbly mess — the smooth mirror has turned rough before your eyes.
Hold up a book to a mirror and try to read it. You can't — the words come out backwards! A flat mirror flips the picture left to right. Whatever is on your left appears on the reflection's right, and whatever is on your right appears on its left. Notice what it does not do, though: it never turns things upside down. Your head stays at the top and your feet stay at the bottom. It is a left–right swap, not a top–bottom one.
You can test it in a second. Raise your right hand and watch the "you" in the mirror. The hand that goes up is on the opposite side — it looks just like a left hand waving back. Wink your right eye and the reflection winks its left. The whole flat world in the glass is quietly reversed, left for right.
Take a look at the front of an ambulance and you'll spot something odd: the word AMBULANCE is painted backwards, reading ƎƆᴎA⅃UBMA. Somebody didn't make a mistake — it's done on purpose, and it's all about mirrors.
When a driver ahead sees the ambulance racing up behind, they see it in their rear-view mirror — which flips everything left to right. A backwards word, flipped again by the mirror, comes out reading perfectly the right way round: AMBULANCE. The driver reads it in a flash and pulls over. A clever trick that uses the mirror's flip to undo itself.
Once you can bounce light exactly where you want it, mirrors become wonderfully useful — they let you see things your eyes could never reach on their own.
A periscope is a tall tube with a mirror tucked at each end, both tilted at a slant. Light from something high up — a ship on the sea, or the far side of a wall — hits the top mirror and bounces straight down the tube. At the bottom it meets the second mirror and bounces again, this time straight into your waiting eye.
With two neat bounces, the view has travelled down and around a corner. That is how a submarine crew can hide safely underwater and still spy a ship on the surface far above — and how you can duck behind a wall and still peek over the top of it, without a single hair showing.