Why is the sky blue? — drill 1a3286

1

The first clueSunlight is lying to you.

Sunlight looks white, doesn't it? That is a trick.

White light is really seven colours holding hands: red, orange, yellow, green, blue, indigo and violet. They are all in the beam already. Your eyes just can't pick them apart while they are huddled together.

Press the beam. Make them let go of each other.

The sky does this same trick — but it only pulls out one colour.

7 colours hiding in white lightthe list is called a SPECTRUM

a beam of sunlight

Press the beam

A raindrop does this to sunlight and you get a rainbow. The sky does it a little bit, every single day.

2

The second clueEmpty air is not empty.

Can you see the air right now? No. So it feels like nothing is there.

But air is made of teeny tiny bits — so tiny that a million of them could have a picnic on the full stop at the end of this sentence. The two kinds of bit with the longest names are called nitrogen and oxygen.

Above your head there is a wall of this stuff about 100 kilometres tall. Scientists call the whole wall the atmosphere. Sunlight has to push through all of it to reach the top of your head.

And when light pushes through bits, it bounces off them. Bouncing light has a science name too:

SCATTERING.

look up

The sky is deepest blue straight overhead

Because that is where you are looking through the thickest, cleanest part of the air wall. Near the horizon the light has travelled further and picked up paler, washed-out colours — which is why sky-blue fades to pale whitish-blue as it gets close to the ground.

Say it with me: scat — ter — ing. It just means bouncing every which way.

3

The big reasonBlue waves are short. Red waves are long.

Light travels in waves, like ripples in a puddle. Each colour makes waves of a different size.

Red waves are long and lazy — the size of a fat earthworm.
Blue and violet waves are short and bouncy — the size of a grain of sand.

Now remember: the air bits are tiny. A long red wave is big enough to step right over them. A short blue wave is not — it trips, bumps and bounces off, again and again, in every direction at once.

That is the whole answer. Blue light gets thrown around the sky by the air, so it arrives at your eyes from every direction — even from the parts of the sky the sun is not in.

This is called Rayleigh scattering, after Lord Rayleigh, who worked it out in 1871. Say "RALE-lee" — like the deer.

blue bounced0 red zoomed through0

← sunlightthe white dots are air bitsyour eyes →

4

Do it yourselfMake a bit of sky in a cup.

You cannot see air bits. But you can see what bouncing light looks like, with milk.

a clear glasswatermilka torcha dark room

Fill the glass nearly full of water.
Ask a grown-up to make the room dark. Shine the torch sideways through the glass.
Look at the water from the side, away from the torch. It looks a bit cloudy, and the far side looks warm and yellow.
Now add one tiny drop of milk. One! Stir it. The milk breaks into millions of little fat bits — pretend they are air bits.
Look from the side again. The water has gone bluish. Now look straight through the water at the torch: it glows orange.
That blue cloudy water is your sky. The orange torch is your sunset. Same trick, in a glass.

Too much milk and it just looks like porridge. Add another drop of water if that happens.

5

Now you know moreWhy sunsets turn red — and why space is black.

Drag the sun. Watch what happens to the sky when the sun gets low.

sunrise sunset

how much air the light must push through1.0×

When the sun goes down, the light has a longer walk.

At midday the sun is above you, so its light only pushes through a short slice of air. Blue bounces out sideways and the sky is blue.

At sunset the light has to slide through a much, much thicker slice of air to reach your eyes — about ten times more. By then almost every bit of blue has already bounced away and gone somewhere else. What is left in the beam is the long, lazy stuff: red, orange and yellow.

A sunset is not a different sky. It is the same blue sky with the blue taken out.

And the same rule explains something else. Go right to the top of the air, where there is almost none left, and the sky turns black — no bits, no bouncing, no blue. Astronauts see a black sky at high noon, with stars in it. The only bright thing is the sun itself.

Dark sky filled with stars, seen from space

from a spaceship

Airless space is black in every direction, because there is nothing there to throw the sunlight around.

6

Five "but wait —" questionsKept on the fridge.

but why not violet?

Violet bounces even harder than blue

True! But the sun sends only a little violet, and your eyes have far more blue sensors than violet ones. Your brain mixes the rest and settles on blue.

and clouds?

Cloud bits are big

A cloud drop is about a hundred times fatter than an air bit. Big things bounce all the colours the same amount, so nothing gets picked out. All the colours together look white.

the sea

The ocean is blue for two reasons

Some of it is the sky's blue bouncing off the surface. But water itself drinks up red light and sends blue light back. That's why a deep glass of water looks faintly blue too.

View of blue sky from an aeroplane window

out of the window

Plane sky is darker

Climb high enough and the sky goes from pale blue to deep indigo, because there is less and less air left above you to do the bouncing.

other planets

Mars has it backwards

Dust, not air, does the bouncing there — and dust is big. It scatters red, so Mars is butterscotch-pink by day and blue around the setting sun.

?

Your turnThree questions. No pressure.

0 / 3

Get all three and you can explain the sky to anyone in your house. Try it out loud — saying it is the best way to find out if you really know it.

Keep going…

tonight

Go outside just after the sun has gone down and look at the patch of sky where the sun was. It is often a deep blue-violet. That is the last of the scattered blue light, still bouncing.

✓

Say it out loudThe whole answer in five sentences.

1

Sunlight is all the colours mixed together.

It only looks white because they are travelling in a bundle.

2

Air is full of tiny invisible bits.

Mostly nitrogen and oxygen, stacked about a hundred kilometres high.

3

Short waves bounce off the bits. Long waves don't.

Blue and violet waves are short, so they get scattered in every direction.

4

So blue arrives at your eyes from everywhere.

Look left, look right, look up — the whole sky is throwing blue at you.

5

And at sunset the light travels through so much air that only red is left.

Same trick, different time of day. That's it. That's the answer.

A

Big word shelfWords from today.

Scattering: Light bouncing off lots of tiny things and flying off in every direction.
Atmosphere: The tall wall of air wrapped around the Earth.
Wavelength: How long one wave is. Red is long. Blue is short.
Spectrum: The line of colours that come out when you split white light.
Nitrogen: The gas that makes about 4 out of every 5 breaths you take.
Oxygen: The other gas in air — the bit your body actually needs.
Rayleigh: Pronounced "RALE-lee". The name of the scattering rule.
Horizon: The line where the sky looks like it touches the ground.