Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one idea about mirrors. The models are simplified to show the idea.
Drag, slide and tap, and see what happens.
Drag yourself closer or farther. How far away is your reflection?
Move the object. The virtual image always sits as far behind the mirror as the object is in front.
Drag the child. What does the mirror twin do?
Drag the flashlight. Is the angle out always the same as the angle in?
Change the angle of incidence and compare it with the angle of reflection, both measured from the normal.
Move the flashlight. Where does the light bounce?
Make the surface rougher. What happens to the rays and the picture?
Roughen the surface: each bump has its own normal, so parallel rays scatter and the image is lost.
Make it bumpy, then smooth. When can you see the face?
The roughness here is a simplified model; real paper fibres are far smaller.
Tap the mirror to pull its layers apart. Which layer does the reflecting?
Glass, silver (or aluminium), often copper, then paint. The light reflects from the metal behind the glass.
Tap the mirror. What is inside?
Layer thicknesses are not to scale: the metal film is far thinner than shown.
Pick a word or type your own. Which words look the same in the mirror?
Try symmetric words like WOW or MOM: the mirror only reverses the axis into the glass.
Pick a word. Does it look backwards in the mirror?
Move the mirrors apart and change how much light each bounce keeps. How many reflections can you see?
Each reflection keeps only part of the light, so the tunnel fades. Change the fraction kept per bounce.
Slide the mirrors. How long is the tunnel?
The light kept per bounce is a made-up example, not a measured value.
When you look in a mirror, there seems to be a whole room in there. But behind the mirror there is only a wall! A mirror works because light bounces.
Shine a flashlight at a mirror and the light bounces off, like a ball off a wall. If it comes in slanted one way, it goes out slanted the other way, just the same. Your eye follows the light back, so you seem to be behind the glass.
Paper bounces light too, but paper is bumpy, so the light goes every which way. A mirror is super smooth, so the light bounces together and the picture is clear. Shiny metal is a super light bouncer.
Long ago, people used shiny black stone and polished metal. Today a factory washes a sheet of glass, adds a very thin coat of silver on the back, and paints over it. The glass in front keeps the shiny metal safe.
A modern mirror is a sheet of flat glass with a very thin film of metal on the back. For a silver mirror, the factory washes the glass, treats it with a tin solution that helps the silver stick, and coats it with silver. A layer of copper may be added on top for durability, and then paint seals the back. (Some mirrors use aluminium instead, put on in a vacuum: see below.)74
The metal does the reflecting. The glass in front protects that fragile film from scratches and tarnish, and gives it a perfectly flat face.7 That's why most household mirrors are called "back-silvered" or "second-surface" mirrors: light passes through the glass first, bounces off the metal, and comes back out.7 (Curious where the flat glass comes from? See how glass is made.)
| Layer | What it's made of | Its job |
|---|---|---|
| Front | Glass | Clear shield and flat surface |
| Bonding layer | A tin treatment (very thin) | Helps the silver stick to glass |
| Reflective layer | Silver, or aluminium | Bounces the light back |
| Protective layer | Copper (in many silver mirrors) | Helps the silver last longer |
| Back | Paint | Seals out air and water |
Mirrors made with aluminium are coated in a different way: the aluminium is heated in a vacuum chamber until it evaporates, and the metal atoms settle on the glass as a thin, even film. It's then protected with waterproof paint.47 Silver is still used for many of the best household mirrors.4
Ever seen a dark, blotchy edge on an old bathroom mirror? That happens when water seeps in behind the glass. The silver slowly tarnishes into dark silver sulfide, which doesn't reflect. To help stop it, wet the cloth, not the mirror, when you clean it (this can't undo old tarnish).4
Stay safe: mirrors are glass. If one cracks or breaks, step away and let a grown-up clean it up, because broken glass is very sharp. And never touch or heat mercury (it is in some old thermometers and very old mirrors): its fumes are poisonous to breathe.11
People have been making mirrors for a very long time. Some of the earliest human-made mirrors were polished obsidian, a black volcanic glass, made in what is now Turkey around 6200 BC. That's more than 8,000 years ago.56 Later, the ancient Egyptians polished copper into mirrors, and many cultures used polished bronze.5
Glass mirrors in the Middle Ages were usually small and often curved.5 In 1507, glassmakers in Italy found that coating the back of flat glass with tin and mercury made startlingly clear mirrors. They were so costly that some nobles sold land to buy one.5 Making them meant heating the coating to drive off the mercury, and mercury fumes are poisonous to breathe.711
The big change came in 1835, when the German chemist Justus von Liebig found a way to deposit a thin layer of real silver onto glass using a chemical reaction. That made good mirrors far easier to produce, and far more common.47
| When | Mirror |
|---|---|
| ~6200 BC | Polished obsidian (volcanic glass), in present-day Turkey |
| Ancient Egypt onward | Polished copper and bronze |
| 1507 | Glass backed with tin and mercury, in Italy |
| 1835 | Glass coated with silver by chemistry (Liebig) |
| Today | Glass with silver or aluminium, protected by paint (often copper too, on silver) |
Everything a mirror does comes from one rule, the law of reflection: the angle at which light hits a surface equals the angle at which it bounces off. Both angles are measured from an imaginary line sticking straight out of the surface, called the normal.12
angle of incidence = angle of reflection
When you look into a mirror, light from your face travels to the glass, bounces off the metal behind it, and comes back to your eyes. Your brain assumes light travels in straight lines (it does, most of the time: see how light travels), so it traces the light straight back and "sees" it coming from a point behind the mirror.2 That's why your reflection seems to stand exactly as far behind the glass as you stand in front of it.12 Physicists call it a virtual image: the light only seems to come from there, so you can't catch it on a screen.3
Mirrors steer light by bouncing it. Lenses steer it the other way, by bending it as it passes through, which is called refraction.2 Cameras use both tricks (how cameras work).
Paper reflects plenty of light. That's why you can see it at all. But up close its surface is rough, so each tiny bump sends light off in a different direction. This scattering is called diffuse reflection. It lets you see the page from any angle, but it scrambles any picture.12
A mirror is smooth, even compared with the tiny size of light waves, so all the light bounces off together at matching angles. This is called specular reflection, from speculum, the Latin word for mirror.12 Shiny metal makes a great mirror surface because it reflects most of the light instead of soaking it up: a fresh metal coating on the front of a mirror can reflect about 90 to 95% of the light.7
Animals use reflection too. Cats have a mirror-like layer at the back of their eyes that bounces light back through for a second chance to be seen, which is why their eyes shine in the dark.12 (More in how cats see.)
It really looks that way. Raise your right hand, and your reflection seems to raise its left. But look carefully: up stays up, and if you point at the wall on your right, your reflection points at that same wall. Nothing swaps sideways.
What a mirror actually flips is front and back, the direction straight into the glass. OpenStax compares it to turning a right-hand glove inside out: you get a left-hand glove. That's why the reflection of a right hand looks like a left hand.3 We describe it as "left and right" because we imagine walking around to stand where the reflection is.
Try it: write a word on your palm with a washable pen and hold it up to a mirror. It looks backwards. Words like WOW or MOM, written in capitals, can look the same both ways: their letters are symmetric and they read the same backwards.
Bend a mirror and the rule stays the same, but every point of the surface tilts a different way. Look into the bowl of a spoon and you'll see yourself upside down.10
Most telescopes, and all large ones, use curved mirrors to gather faint light and bring it to a focus. Mirrors are lighter than lenses, and a big mirror is much easier to make near-perfect than a big lens.10 The James Webb Space Telescope's main mirror is 6.5 meters across, built from 18 six-sided segments of beryllium, each about 20 kilograms.8 The segments are coated in gold, because gold reflects infrared light very well, and the coating is only about 100 nanometers thick, one ten-thousandth of a millimeter.89
Stand two mirrors facing each other and put a small object between them. You'll see a row of reflections stretching away, because each mirror reflects the other's image, again and again.3 The tunnel doesn't go on forever in practice: every bounce loses a little light, so the far images fade and shrink away. In the last lab above, the percentage of light kept per bounce is a made-up example you can change, not a measured value for any particular mirror.
A second easy test: shine a flashlight at a mirror at an angle in a dim room, then at a sheet of paper. On the mirror you get one bright bounced spot on the wall. On the paper, the light spreads everywhere.1
A factory washes a sheet of very flat glass. For a silver mirror, the glass is treated with a tin solution so the metal will stick, then coated on the back with a very thin film of silver; a layer of copper is often added, and paint seals and protects it. Aluminium mirrors are made differently: the metal is coated onto the glass inside a vacuum chamber, then sealed with paint.
Most household mirrors are glass with a thin layer of silver or aluminium on the back, protected by paint (many silver mirrors also have a copper layer). The metal does the reflecting. The glass protects it and gives it a perfectly flat face.
Plain glass lets most light through. Once a smooth metal film is added to the back, the light that passes through the glass hits the metal and bounces back, so you see a sharp reflection.
A mirror is so smooth that light bounces off it in an orderly way: the angle the light comes in at equals the angle it leaves at. Your eye follows the bounced light back in a straight line, so the image seems to be behind the glass.
Mirrors are mostly glass, and glass is heavy. A big mirror is a big sheet of glass, often thick, in a solid frame. The metal layer itself is extremely thin and adds almost no weight.
No. A mirror flips front and back, the direction straight into the mirror. Up stays up and a hand pointing at a side wall still points at that wall. We read it as a left-right swap because we imagine turning around to stand where the reflection is.
Some old mirrors were. From the 1500s, glassmakers coated glass with a mix of tin and mercury. Mercury fumes are poisonous, and in 1835 the chemist Justus von Liebig found a way to coat glass with silver instead. Modern mirrors use silver or aluminium. Never touch or heat mercury: its fumes are poisonous to breathe.