Why Is the Sky Blue — and Red at Sunset?

We look at the sky thousands of times without asking: why blue specifically? And why does it turn a gorgeous orange-red at sunset? The answer is a beautiful story about light and air.

White light isn't really white

The sunlight we see as white is actually a mix of every color of the spectrum: red, orange, yellow, green, blue, indigo, violet. A glass prism separates them into a rainbow. Each color travels at a different wavelength: red is long, blue is short.

The encounter with air molecules

As sunlight enters the atmosphere, it collides with tiny molecules of nitrogen and oxygen. These molecules scatter short-wavelength colors (blue and violet) far more than long ones. This is known as Rayleigh scattering.

As a result, blue light spreads in every direction across the sky, so your eye sees it coming from everywhere — hence the blue. (Violet scatters even more, but our eyes are more sensitive to blue and the Sun emits more blue, so blue wins.)

Why does sunset turn red?

At sunset the Sun sits near the horizon, so its light travels a much longer path through the atmosphere before reaching you. Along that long path, blue is scattered away entirely, and only the long wavelengths — red and orange — reach your eye. That's why the sky glows warm at sunrise and sunset.

A simple thought experiment

Picture blue as a small fast ball bouncing off every obstacle, and red as a big heavy ball rolling straight through. The more obstacles (more air at sunset), the more blue scatters and the more red survives. The same physics explains why the Sun itself looks yellow to us, though it is white in space.