Everyday science

Why is the sky blue — and why are sunsets red?

Sunlight looks white, but it's actually a mix of every colour. The sky's colours are the story of what the air does to that light on its way to your eyes.

White light is secretly a rainbow

Pass sunlight through a prism and it fans out into red, orange, yellow, green, blue, violet. Each colour is a wave, and the colours have different wavelengths: red is long and lazy, blue and violet are short and tight.

Why the daytime sky is blue

Earth's air is full of tiny gas molecules. When sunlight hits them, the light gets scattered — bounced off in all directions. But the air doesn't scatter all colours equally. It scatters short wavelengths (blue) far more strongly than long ones (red). This is called Rayleigh scattering.

So blue light gets ping-ponged around the whole sky and comes at your eyes from every direction. Look anywhere up, and you're seeing scattered blue. The sky is blue because the air is, in effect, glowing blue with redirected sunlight.

(Violet is scattered even more — but the Sun gives off less violet, and our eyes are less sensitive to it, so blue wins.)

Why sunsets turn red

At sunset, the Sun sits low on the horizon, so its light has to travel through much more air to reach you — a long, slanting path instead of a short overhead one.

Over that long journey, almost all the blue has been scattered away *before* the light gets to you. What's left to come straight through are the long wavelengths — the reds and oranges. So the Sun and the clouds around it blaze warm.

Bonus: why it can go fiery

Dust, smoke, and pollution add bigger particles that scatter light too, which is why the most spectacular red sunsets often follow a dusty day or a distant wildfire — and why sunsets after big volcanic eruptions have been some of the most vivid in history.

So it's all one effect seen two ways: overhead, short blue light scatters toward you; at the horizon, only long red light survives the trip. Same Sun, same air — just a different path.