Why Is the Sky Blue?

Step 1 of 5

Sunlight is every colour mixed together.

It looks white, but a prism splits it into red, orange, yellow, green, blue and violet. All of them arrive from the Sun together.

Step 2 of 5

Colours are waves of different sizes.

Red light has long, lazy waves. Blue light has short, tight ones. That size difference is what makes the sky blue.

Step 3 of 5

Air molecules bounce the short waves hardest.

Nitrogen and oxygen molecules are just the right size to knock blue light sideways. Red light mostly sails straight past them.

Step 4 of 5

So blue comes at you from the whole sky.

The scattered blue has bounced around and now arrives from every patch of sky, not just from the Sun's direction. That is what you see overhead.

Step 5 of 5

At sunset the light travels much further.

Low sunlight crosses far more air, so nearly all the blue is scattered away before it reaches you. What is left is red and orange.

Violet scatters even more than blue, but our eyes are far less sensitive to it — so we see blue.