Why is the sky blue? And why are sunsets red?

"Why is the sky blue?" — this is perhaps the most common question of childhood. The answer is as beautiful as it is simple, once we understand a thing or two about light. The same science also explains why sunsets are red-orange and why clouds look white.

Seven colours hidden in sunlight

Sunlight looks white to us, but in fact it contains all the colours of the rainbow mixed together. Each colour has its own wavelength — red has the longest and violet-blue the shortest.

Air molecules scatter the colours

The Earth's atmosphere is made of very small molecules of gases such as nitrogen and oxygen. When sunlight hits them, it is scattered in all directions. This is called scattering. In the 19th century the British scientist Lord Rayleigh showed that small particles scatter colours of shorter wavelength much more. That is why it is called Rayleigh scattering.

Blue light is scattered several times more than red light. In the daytime, whichever direction of the sky we look at, this scattered blue light reaches our eyes from there — and the sky looks blue.

So why is the sky not violet?

Violet is scattered even more than blue, yet the sky does not look violet. There are three reasons:

Sunlight contains less violet than blue.
Part of the violet light is absorbed high up in the atmosphere.
Our eyes are more sensitive to blue than to violet.

Did you know? On a sea voyage in 1921, India's C. V. Raman saw the deep blue of the Mediterranean and wondered why the sea is blue. The research that began with this curiosity later led to the discovery of the "Raman effect".

Why are sunsets red-orange?

In the morning and evening the Sun is near the horizon, so its light has to cross a much thicker layer of atmosphere to reach us. On the way, almost all the blue light is scattered off in other directions, and mainly red, orange and yellow light reaches us. Particles of dust and smoke make the colours of the sunset even deeper.

Why are clouds white?

The water droplets in clouds are much bigger than air molecules. Big particles scatter all colours almost equally, so clouds look white or greyish-white. When clouds become very thick, less light gets through them and they start to look dark — a sign of rain.

The sky on other worlds

The Moon has no atmosphere, so light is not scattered at all — the sky looks black even in the daytime. On Mars, the dusty atmosphere makes the daytime sky look a pale brownish-pink, and at sunset a blue glow appears around the Sun.