Why the Sky Is Blue

A question worth looking up for

Lie on the grass on a clear day and look up. The sky is a deep, even blue from one side to the other. But the air is invisible — you can see straight through it — so where does the colour come from? The answer is a piece of physics about light called scattering.

Sunlight is a rainbow in disguise

The first clue is that white sunlight is not really white. It is a mixture of all the colours of the rainbow blended together. When raindrops or a glass prism split sunlight apart, you see the hidden colours come out — exactly what happens in how rainbows form.

Each colour of light is a wave, and the colours have different wavelengths:

• Red light has the longest waves.
• Blue and violet light have the shortest waves.

That difference in wavelength is the key to the whole puzzle.

Air bounces blue light around

The air is full of tiny molecules of nitrogen and oxygen gas. When sunlight streams through, these molecules scatter the light — they catch a bit of it and fling it off in new directions.

Here is the important part: the smaller the wavelength, the more it gets scattered. Short blue waves get bounced around far more strongly than long red waves. Scientists call this Rayleigh scattering, after the physicist Lord Rayleigh who explained it.

So as sunlight passes overhead, the blue part is scattered out of the beam and sprayed across the entire sky in every direction. Wherever you look, scattered blue light is reaching your eyes — and so the sky looks blue all over. The red and yellow light mostly travels straight through, which is why the Sun itself looks pale yellow-white.

Why the same physics paints a red sunset

Now wait for the evening. At sunset the Sun is low on the horizon, so its light has to travel through a much thicker slice of atmosphere to reach you.

Along that long path, almost all the blue light gets scattered away before it arrives. What is left is the longer-wavelength red and orange light, which passes through more easily. That remaining warm light is what glows across the sky and lights up the clouds. The same scattering that makes a noon sky blue makes a sunset red — it just depends on how much air the light crosses.

A clue from the Moon

If scattering by air makes the sky blue, then a world with no air should have no blue sky at all. And that is exactly what astronauts found on the Moon. The Moon has almost no atmosphere, so there is nothing to scatter the sunlight. Even with the Sun blazing, the lunar sky stays pitch black. This is strong evidence that our blue sky is made by the air, not by the Sun.

Try it yourself! 🧪 (safe version)

You can recreate a blue sky and a red sunset in a glass of water.

You need: a clear glass or jar of water, a torch (flashlight), and a few drops of milk.

1. In a dark room, shine the torch through the plain glass of water from the side. The beam looks white.
2. Stir in just a few drops of milk and shine the torch through again, watching from the side. The water now glows faintly blue — the tiny milk particles scatter blue light, just like air molecules do.
3. Now look through the glass straight at the torch. The light that comes through looks orange or red — the blue has been scattered away, leaving the warm colours, exactly like a sunset.

The milk droplets are standing in for the gas molecules of the sky. To explore how the different colours of light behave once they reach you, see how we see colour and colours and filters.