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Look up on a clear day

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and the sky is blue.

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But sunlight itself looks white,

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and space is black.

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So where does all that blue come

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from, and why does the same sky turn orange

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and red at sunset?

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The short version fits in one sentence:

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air scatters blue light far

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more than red light.

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The rest of this video unpacks

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what that means, why it happens,

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and how the same idea explains sunsets.

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First, sunlight is not one colour.

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It is a mix of every colour

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of the rainbow, travelling together.

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When they arrive at your eye

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in roughly equal amounts,

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your brain reads the mix as white.

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Each colour is light

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with a different wavelength, the distance

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from one wave crest to the next.

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Red waves are about 700 nanometres long,

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a nanometre being a millionth of a millimetre.

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Green is about 530, blue about 450, and violet,

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the shortest we can see, about 400.

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A prism proves the mix is there.

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It bends short waves

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a little more than long ones,

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so white light fans out into a rainbow,

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traditionally named as 7 colours.

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So if sunlight holds every colour,

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what happens to it on the way down?

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Before sunlight reaches you, it passes

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through miles of air.

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Air is mostly nitrogen

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and oxygen molecules, far smaller

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than a wavelength of light.

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When light hits them, some

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of it bounces off in new directions.

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That is called scattering.

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Here is the key.

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For particles much smaller than the wavelength,

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the amount of scattering rises very steeply

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as the wavelength gets shorter.

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Physicists call it Rayleigh scattering,

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after the scientist

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who worked it out in the 1800s.

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How steep?

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Blue light is scattered about 5.9 times

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more strongly than red, and violet,

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the shortest wave, about 9.4 times.

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So as sunlight crosses the sky, blue

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is constantly knocked sideways,

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while red mostly carries straight on.

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Now picture any patch

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of sky away from the sun.

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The light reaching you from there is light

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that was scattered toward your eye.

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And scattered light is mostly blue.

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That is why the whole dome

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of the sky glows blue.

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But if violet scatters even

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more, why isn't the sky violet?

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Violet does scatter even more than blue.

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Three things work against it.

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Sunlight contains less violet

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to begin with.

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Some of it is absorbed high

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in the atmosphere.

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And your eyes are simply much

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less sensitive to violet than to blue.

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Your eyes read colour

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with three kinds of cone cells.

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The scattered skylight excites the blue ones

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and a little of the green ones,

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and your brain adds that up

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as a pale sky blue rather

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than a deep violet.

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The same idea explains sunsets.

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When the sun is low,

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its light travels through much more air

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to reach you than at noon.

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Along that long path,

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most of the blue gets scattered away

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before it arrives.

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What survives the trip is the light

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that scatters least: oranges and reds.

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So the sun and the clouds near

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it glow warm, while the blue

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that was scattered out is lighting up

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the sky for people farther west.

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Dust, smoke

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and larger droplets change the picture.

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Bigger particles scatter all colours

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more evenly, which is why haze looks white

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or grey, and why smoke from distant fires

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can make sunsets especially deep red.

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So the sky is blue

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because air scatters short blue waves far

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more than long red ones,

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and sunsets are red because a long path

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through the air strips the blue away.

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Got a question,

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or spotted something to correct?

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Tell us in the comments.