A child on a grassy hill points up at a big blue sky with a bright golden sun and fluffy clouds.

Why is the sky blue?

Sunlight looks white. The air is clear — you see right through it. So where does all the blue come from? Let's find out.

First clue: sunlight only looks white. Tap it and see what's really inside.

tap the white light

Next clue: the air is full of tiny specks. When sunlight hits them, the blue scatters much more strongly than red. More red keeps travelling in its original direction.

A golden sunbeam hits a cluster of tiny air specks: blue light splatters off them in every direction while a red beam passes straight through and keeps going.

But why blue and not red? Blue light has shorter waves than red. Air molecules scatter those shorter waves more strongly. These big drawn waves help us compare — real air molecules are much smaller than either wave.

A small tight blue wave crashes into tiny specks and scatters in dotted arcs, while a big slow red wave below rolls straight past the same specks.

So all that scattered blue, coming at you from every direction — that's your blue sky!

But what if light crosses a LOT of air? Drag the slider and count how many chances the blue gets to bounce away.

air to cross: a little
thinthick

More air gives blue more chances to bounce away. The colour that gets out stays hidden — use the clues to predict it.

Look how far the light has to travel.

Two panels of the same child on a hill: a high noon sun sends a short beam straight down through a little air, while a low sun sends a long beam sideways through much more air to reach the same child.

At noon the light takes a short hop through the air. At sunset it has to cross a HUGE amount of air… 🤔

You just saw: thicker air steals the blue.

So when the sun drops low and its light crosses all that extra air — what colour does the sun turn?