Why Is the Sky Blue — and Why Does It Turn Red at Sunset?

Hold a glass of water up to the light. It's perfectly clear. The light passes straight through. Now add a tiny drop of milk and look again — the glass has a faint blue tint, and the light exiting the far side looks slightly orange.

You've just seen the same process that colours the sky.

Sunlight isn't actually white. It's all colours at once — a complete spectrum from violet to red — that blend together into what we perceive as white. When that light enters Earth's atmosphere, something interesting happens: it hits molecules of nitrogen and oxygen, and it scatters.

But not equally. Blue light, with its short wavelength, scatters roughly 5.5 times more than red light. Every time a blue photon hits an air molecule, it bounces off in a random direction. Red photons mostly travel straight through.

The result: blue light ends up bouncing around the entire sky, reaching your eyes from every direction. No matter which way you look (except directly at the sun), you're seeing scattered blue light. The sky appears blue.

Now take that same light and make it travel through much more atmosphere. At sunset, the sun is near the horizon — its light cuts diagonally through the atmosphere rather than straight down, passing through roughly 38 times more air. By the time it reaches you, almost all the blue has already scattered away in other directions. Only the stubborn red and orange wavelengths, which scatter least, survive the journey.

The same thing happens in your glass of milk. The milk droplets scatter blue light sideways — so you see blue from the side. The red light passes straight through to the other end.

What's remarkable is that blue skies and red sunsets are the same phenomenon, just at different distances. The atmosphere isn't doing two different things. It's just doing one thing from two angles.