Why is the sky blue?

On a clear day the sky is blue and the Sun in it is yellow. Yet in space around the Earth the sky is black. So where does the blue come from?

Is it reflecting the sea?

People often say the sky is blue because it reflects the sea. But over a desert, hundreds of kilometres from any coast, the sky is just as blue. It’s rather the other way round: the sea reflects the sky.

Sunlight is a mixture of colours

Light from the Sun is actually a mixture of all the colours of the rainbow. A glass prism shows it: it splits the light into colours from red to violet.

Each colour has a different wave

Light is a wave, and colours differ in their wavelength. Red light has long waves, blue and violet light short ones. All of them are tiny, about half a thousandth of a millimetre.

Air scatters light

Air is made of tiny molecules of nitrogen and oxygen, a thousand times smaller than a wave of light. Most light flies straight past them, but now and then a molecule sends a bit of light off in another direction. This is called scattering.

Blue gets scattered more

Light makes a molecule vibrate, and the molecule then shines out to the sides itself. Short blue waves make it vibrate faster, and the faster it vibrates, the more light it scatters. So blue is scattered about five times more than red, which passes through the air almost entirely in a straight line.

That’s why the whole sky glows

Wherever you look away from the Sun, you see light that air molecules have sent your way. And that light is mostly blue, which is why the whole sky glows blue. Without air the sky would be black even in the daytime, as it is on the Moon.

So why is the Sun yellow?

The blue you see in the sky is missing from the beam coming straight at you from the Sun. With some of its blue gone, sunlight looks yellowish. Astronauts in orbit see a white Sun, because they don’t look at it through air.

Then why isn’t it violet?

Violet light has an even shorter wave, so it scatters even more. But sunlight contains less of it than blue, and our eyes are not very sensitive to it. So from the mixture of scattered colours, we see blue.

Why is the sunset red?

In the evening sunlight comes in at a slant and travels much further through the air than at noon. Almost all the blue is scattered along the way, and what reaches you is mostly orange and red.

Try it yourself

Put a few drops of milk into a tall glass of water and, in a dark room, shine a torch through it from the side. Seen from the side, the water will look bluish, and the patch of light on the wall behind the glass will be orange. The droplets of milk scatter light much as the air does.

So why is the sky blue?

Sunlight is a mixture of colours. Air scatters short blue waves far more than long red ones, so blue reaches us from all over the sky. Some of it is then missing from the direct beam from the Sun, which is why the Sun is yellow. In the evening almost all of it is missing, and red is what’s left.