Light & colour

Sky on fire, quietly

What Are the Northern Lights?

Earth's magnetic field turns the solar wind aside but funnels some of it down at the poles, where it lights up atoms 100 km overhead like a neon sign.

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Three pieces of evidence

How Do We Know the Universe Had a Beginning?

Nobody watched it happen. Galaxies receding, a faint warmth in every direction, and the recipe of the gas all point back to the same hot, dense moment.

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Gravity turned all the way up

What Is a Black Hole?

Pack matter tightly enough and escaping it would mean travelling faster than light. Nothing can, so nothing leaves — not even the light itself.

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Join the dots

What Are Constellations?

The stars in a constellation are nowhere near each other. The picture exists only from where we stand, and only for the few thousand years we are here.

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A very old puzzle

Why Is the Night Sky Dark?

With endless stars the sky ought to blaze. It does not, because the universe has a finite age and is stretching light beyond what any eye can catch.

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More time in the light

Why Are Days Longer in Summer?

The spin never varies — 24 hours, every day. The tilt changes how much of that spin you spend on the sunlit side, which near the poles can mean all of it.

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A spinning Earth story

Why Day Turns Into Night

Night is not the Sun switching off. It is your side of a spinning Earth turned away from it, waiting its turn to come back round.

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Three things in a line

What Happens in an Eclipse?

The Sun is 400 times wider than the Moon and 400 times further off, so the two look the same size — which is why a total eclipse fits exactly over it.

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Watching for a tiny dip

How Do We Find Planets Around Other Stars?

Almost no planet round another star has been seen. They are found by the tiny, precisely repeating dip in starlight as the planet crosses in front.

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The missing wobble

How We Proved the Earth Moves

The best argument against a moving Earth was that the stars never shifted — a good argument, until telescopes got precise enough to catch the shift.

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Dark lines in the rainbow

How We Know What Stars Are Made Of

A philosopher called it permanently unknowable. Every element absorbs its own exact colours, so a star's rainbow spells out its ingredients.

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You would not notice for eight minutes

What If the Sun Went Out Tonight?

The light takes eight minutes and twenty seconds to arrive, and so would the news — after which the freeze takes days rather than seconds.

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A distance, not a time

What Is a Light Year?

A light year sounds like a duration and is really a distance: the 9.5 trillion kilometres light covers in a year. Every glance up looks back in time.

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A rusty planet

Why Is Mars Red?

Mars is red for the same reason an old nail is: its iron-rich dust has rusted. A whole planet the colour of something left out in the rain.

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You live inside it

What Is the Milky Way?

We sit 26,000 light years out in a disc 100,000 light years across and barely 1,000 thick, so looking along the plate turns countless stars into a stripe.

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It was never a night object

Why Can You See the Moon in Daytime?

It rises about fifty minutes later each day on its own timetable. A full Moon is a night object, but a half Moon stands high in broad afternoon.

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Too far away to fall behind

Why Does the Moon Follow the Car?

A hedge sweeps past in a second and a far hill barely stirs. The Sun, the planets and every star hold their place on a journey for the same reason.

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Same ball, different view

Why the Moon Changes Shape

Exactly half the Moon is lit at every moment. The phase is only the angle you happen to be looking at that half from, and one lap takes 29.5 days.

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A star nursery

What Is a Nebula?

A nebula is a vast cloud of gas and dust: both the raw material that stars condense out of, and the wreckage they scatter when they die.

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Colour is temperature

Why Are Some Stars Red and Others Blue?

A star's colour is a thermometer you can read from a hundred light years away: red is the cool end, blue-white the hot end. It says nothing about size.

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The air is doing it

Why Do Stars Twinkle but Planets Do Not?

The twinkling happens in our restless air, not in the star. A star is a single point of light for the air to bend; a planet is wide enough to average out.

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A sun, seen from far away

What Is a Star?

Nothing in a star burns — there is no oxygen out there. Its own weight crushes the middle to millions of degrees, and the light leaks out of that.

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A very slow journey with a fast ending

The Last Eight Minutes Were the Easy Part

Energy from the Sun's core takes an enormous time to bounce its way out before the eight-minute run to Earth — and that delay is very hard to pin down.

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A bigger eye

How Do Telescopes Work?

A telescope is a bucket for light, and the wider the bucket the more it catches — far more starlight than a pupil a few millimetres across ever could.

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Light, sorted

Where Do Colours Come From?

White light is every colour at once. An object's colour is the one slice it refuses to absorb and throws back at your eyes — the leftovers, in other words.

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Bouncing all the way

How Does Light Go Round a Bend?

Light strikes the inside wall of a glass fibre so shallowly that it reflects perfectly, thousands of times a metre, tracing the cable round bends.

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Rømer, 1676

How We First Timed Light

Light was thought to be instant until Jupiter's moon kept running late, arriving behind schedule whenever Earth was further away.

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Light marching in step

What Makes a Laser Different From a Torch?

A torch throws every colour every way with the waves unrelated. A laser is one colour, one direction, and every wave stepping in time.

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Bending light on purpose

How Do Lenses Work?

Curved glass slows light by different amounts across a beam, and uneven slowing makes the beam turn — towards a point, or away from one.

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A very smooth wall

How Do Mirrors Work?

Every surface bounces light. A mirror is simply flat enough to bounce every ray without scrambling the arrangement, so the picture survives the trip.

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Same family, different size

Is Radio Really a Kind of Light?

Radio is genuinely light, just stretched to waves metres long instead of fractions of a millimetre. That length is why it passes through walls.

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Light coming back

Why Can You See Yourself?

Light bounces off a smooth surface at exactly the angle it arrived, and your brain reads the returning rays as an object standing behind the glass.

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Light changing lanes

Why Does a Straw Look Bent?

Light changes speed crossing from water into air, and changing speed at an angle makes it turn. Your brain assumes straight lines and misplaces the straw.

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Light in a straight line

Why Do Shadows Happen?

Light travels in straight lines and cannot bend around things. A shadow is simply the gap left where something stood in the way of those lines.

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Sun overhead

Why Are Shadows Short at Noon?

Shadow length is a reading of the Sun's height, nothing more. The Sun is highest at midday, so that is when your shadow is at its stubbiest.

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About a hundred watts of it

You Are Glowing Right Now

Warm objects radiate light whose colour depends on their temperature — yours is infrared, invisible to us and perfectly obvious to the right camera.

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Breaking up the outline

How Does Camouflage Work?

Camouflage is less about matching a colour than about destroying a recognisable outline and cancelling the animal's shadow. It fails the moment it moves.

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You are what you eat

Why Are Flamingos Pink?

The pink is not theirs. Pigment from the algae and shrimp they eat is carried into each feather as it grows, so the colour is eaten first.

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Sunscreen made at home

Why Do Hippos Sweat Red?

A hippo has no fur, so it oozes a red secretion that is sun cream and antiseptic at once, and stays submerged through the heat of the day.

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From somewhere it has never been

How Does a Pigeon Find Its Way Home?

A pigeon works out where it is from magnetism and smell, then takes a bearing from the sun — so a shifted body clock sends it confidently wrong.

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Sticky feet and a very fast eye

How Does a Fly Walk on the Ceiling?

A fly lands upside down on clawed, glue-footed pads, sees your hand coming far sooner than you expect, and tastes whatever it stands on.

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A warning, not a decoration

Why Are Ladybirds Red?

That red is a warning, backed by a foul fluid the beetle leaks from its knees. It works because every ladybird has agreed to look the same.

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A very old rule, badly broken

Why Do Moths Fly Into Lights?

A moth is not drawn to your porch light. It is holding a steady angle to a distant one, and a close lamp turns that rule into a spiral.

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A whole second shift

Why Do Some Animals Only Come Out at Night?

Night is a shift, not an empty time: cooler air, fewer eyes on you, and food nobody else is taking, for animals built to sense the dark.

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Skin that works like a screen

How Does an Octopus Change Colour?

An octopus matches a rock in under a second by squeezing millions of tiny colour bags open and shut — and it is colour-blind while doing it.

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Hearing in three dimensions

How Do Owls Hunt in the Dark?

An owl's ears sit at deliberately different heights, so a sound reaches one a fraction before the other. That gap is enough to locate a mouse in the dark.

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Orange that disappears

Why Are Tigers Striped?

A tiger is bright orange and nearly invisible, because stripes break up its outline and the deer it hunts cannot tell orange from green.

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The oldest argument in the field

Why Do Zebras Have Stripes?

People argued about zebra stripes for a hundred and fifty years. The best answer involves biting flies, not lions — plus blurring the herd.

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A very fast crowd

Why Does a Pond Turn Green?

Sunlight is free, so stray fertiliser is what sets the limit — remove it and a handful becomes millions in a week, shading the pond floor dead.

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A planned demolition

Why Do Leaves Change Colour?

A tree times it by the length of the night, so a warm autumn still turns on schedule. The leaf never falls, either: a layer of cells forms and lets go.

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Watching blood being cleared up

Why Does a Bruise Change Colour?

A bruise is blood spilled under unbroken skin, and its parade of colours is that blood being taken apart one stage at a time.

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The colour it refuses

Why Are Leaves Green?

Green is the one colour chlorophyll cannot use, so it is thrown back at you. The yellows sit underneath all summer, until autumn strips the green away.

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Moving the focus point

How Do Glasses Help You See?

Sharp vision means light landing exactly on the retina, and a millimetre of extra eyeball is enough to blur it. A lens moves the focus back again.

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Building food from air

How Do Plants Eat Light?

Plants build sugar out of air, water and light. A tree is made mostly of air — its carbon comes from carbon dioxide, not from the soil it stands in.

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Green, but not a plant

Is Seaweed a Plant?

Land plants all descend from one green ancestor. Seaweeds come from three separate lines, and many lift their fronds to the light on gas bladders.

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Light in, picture built

How Do Your Eyes See?

Eyes turn light into nerve signals. The picture you see is assembled afterwards by the brain — patched, corrected, and rather less direct than it feels.

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More than a wrapper

What Does Your Skin Do?

Skin is four things at once: a barrier that keeps the world out, a thermostat, a sense organ, and a surface that entirely replaces itself as it wears away.

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Borrowing someone else's trunk

Why Do Vines Climb?

Building a trunk is enormously expensive, so a vine grows thin and fast, grabs a support somebody else built, and spends the rest on leaves.

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Wide, narrow, wide

How Does a Shop Scanner Read a Barcode?

Guard bars differ at each end, so a label scanned backwards still reads. The stripes carry no price at all — that lives in the shop's own computer.

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Counting light in tiny buckets

How Does a Photo Get Caught?

A photo is a count of light: the lens focuses it, the shutter times it, and a grid of tiny wells measures how much landed in each one.

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A wire too hot to stay dark

How Does Electricity Become Light?

Only the filament is thin, so the current squeezes there and pours out white light near 2,500°C. An LED skips the heat entirely.

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A hole, a lens and a very short while

How Does a Camera Freeze a Moment?

A photograph is light let in for a fraction of a second and recorded before anything in front of the lens had time to move and blur it.

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Dots, fired one at a time

How Does a Printer Place Ink So Exactly?

A resistor boils a speck of ink for a few millionths of a second and the steam bubble shoves one drop out. Black is added because three inks make mud.

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A light you cannot see

How Does a Remote Control Reach the Television?

Infrared will not pass through a sofa but bounces nicely off a pale ceiling, which is why aiming a remote at the wall so often works anyway.

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Millions of tiny lights

Why Are Pictures Made of Dots?

Nothing on a screen ever travels anywhere. Red and green side by side read as yellow, and your eye cannot separate them, so it reports the mixture.

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Light knocking electrons loose

How Does Sunlight Become Electricity?

Each packet of sunlight can knock one electron loose, and a solar cell is layered so that every freed electron flows the same way out.

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Not through the sky

The Internet Runs Along the Sea Floor

About 99 per cent of international internet traffic crosses the ocean floor by fibre-optic cable rather than satellite — and ships have laid it since 1858.

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Radio you cannot hear

How Does the Internet Cross a Room?

Wifi is radio. Your data rides on a wave wobbled in a pattern both ends agreed to read, and every device sharing that channel waits its turn.

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A colour that was chosen

Why Are Carrots Orange?

Wild carrots are pale and thin. The orange root is a few centuries old, bred deliberately for the pigment it packs away underground.

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Making a colour hold on

How Does Colour Stay in Cloth?

Getting colour onto cloth is easy; surviving twenty washes is the hard part, so a dye bonds to the fibres rather than sitting on the surface.

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The metal that will not change

Why Is Gold So Special?

Almost every metal dulls or rusts. Gold sits unchanged for thousands of years, bends easily, is genuinely rare, and was made in dying stars.

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Some colour dissolves, some just sits there

Why Does Ink Stay on the Paper?

Sunlight pulls dye molecules apart far faster than it harms a solid pigment speck, which is why the archive keeps pigment and the receipt fades.

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Three jobs in one tin

What Is Paint Made Of?

Every paint is the same three things: coloured powder, a glue to hold it on the wall, and a liquid that thins it and then leaves once it dries.

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A stem pretending to be a root

Why Do Potatoes Go Green?

A potato is not a root but a swollen underground stem, so light turns it green and triggers the bitter defence that comes with the colour.

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A stem that grows crooked on purpose

Why Do Sunflowers Follow the Sun?

It swings back overnight and faces east before the sun appears, which means it is running on an internal clock rather than simply chasing the light.

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Light finds nothing to grab

Why Can You See Through Glass?

Glass electrons can only absorb one exact size of bite and visible light is too small, so a window stops sunburn but never the view.

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A shared pool of electrons

Why Is Metal Shiny?

Polishing adds no shine at all — rough silver bounces back just as much light, only in every direction. Gold looks warm because it eats a little blue.

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Light getting bounced

Why Is the Sky Blue?

Air molecules knock short blue waves sideways about ten times more readily than red ones, so blue arrives at your eyes from every patch of sky at once.

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The line the sunlight hits squarely

What Is the Equator?

The equator is the line halfway between the poles, where sunlight lands most directly and days stay the same length all year.

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Raindrops as prisms

How Does a Rainbow Work?

Every raindrop bends sunlight, splits it and throws it back at a fixed 42-degree angle. The bow is a circle drawn on the sky opposite the Sun.

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An old rhyme, half right

Does a Red Sky Really Predict the Weather?

A red sunset means clean dry air off to the west. That predicts tomorrow only where the weather reliably arrives from the west — Britain included.

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Water eats red light

Why Is the Sea Blue?

Ten metres down there is essentially no red light left at all, and a calm surface still mirrors the sky, which is why the colour varies so much.

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Ice with air in it

Why Is Snow White and Fluffy?

A snowflake is ice grown into a six-armed crystal, and a snowdrift is mostly the air between them. The whiteness is countless surfaces bouncing light.

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Light not getting through

Why Do Storm Clouds Look Dark?

Every bounce costs a little light, so through kilometres of droplets almost none reaches the base. The grey is a measure of how much water is overhead.

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Light taking the long way

Why Are Sunsets Orange?

Near the horizon a sunbeam crosses roughly forty times as much air as at noon, and almost all of its blue is scattered away before it ever reaches you.

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Every mix absorbs more

Why Does Mixing Paint Make Brown?

Every paint absorbs some of the light landing on it, so mixing paints absorbs more and more until almost nothing bounces back. Mixing light is the reverse.

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Colours bending differently

What Does a Prism Do?

White light is a bundle of colours and each bends by a slightly different amount, so a prism's angled faces add those bends up until the colours separate.

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Blocking light, close and far

How Do Shadow Puppets Work?

Light travels in straight lines and spreads out as it goes, so a hand held near the lamp blocks a far wider cone and throws an enormous shadow.

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Light bouncing off two surfaces

Why Are Soap Bubbles Colourful?

No pigment is involved. Light reflects off both sides of a film a thousandth of a millimetre thick, and a different colour cancels at each thickness.

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A shadow swinging with the sky

How Does a Sundial Tell Time?

The shadow swings fifteen degrees an hour because Earth turns fifteen degrees an hour. Aim the pointer at the pole and the hours come out evenly spaced.

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