Why is the sky blue and the sunset red?

The principle

Step 1: Sunlight is all colours mixed. White sunlight is red, orange, yellow, green, blue and violet together, like the colours you see in a rainbow. Each colour is a light wave of a different length: red about 700 nanometres, blue about 450 (a nanometre is a millionth of a millimetre).

Step 2: Air bounces blue the most. Air is mostly nitrogen and oxygen molecules, far smaller than a light wave. When light hits one, a little of it is bounced off in a new direction (this is called scattering). Short waves are bounced far more than long ones: halve the wave length and you get 16 times the bouncing (2 × 2 × 2 × 2, scientists write it as 1/λ⁴). Blue’s wave is about 1.5 times shorter than red’s, so blue is scattered about 5 to 6 times more than red. Like waves in a harbour: short ripples bounce off the small posts, long swells mostly roll on past.

Step 3: Why the sky is blue. With the sun high, its light crosses the air the short way. Along the way a bit of blue is bounced out of the beam in every direction, so wherever you look in the sky, some bounced blue is heading your way. That is the blue sky. The sun itself still looks almost white, because only a small share of its blue has been lost.

Step 4: Why sunsets are red. When the sun is on the horizon, its light skims sideways through the air and crosses about 40 times as much air as when it is straight overhead (about 38 times, by the standard tables). Over that long trip nearly all the blue and much of the green is bounced away, so what is left in the beam, and in the sun you see, is orange and red. Dust, smoke and sea salt in the low air can make it even redder.

Step 5: Putting it together. Short waves bounce most. Sun high, little air: you see the bounced blue all over the sky. Sun low, about 40 times the air: the blue is gone before it arrives and only red gets through.

Why not violet? Violet is bounced even more than blue, but sunlight has less violet to begin with, some is soaked up high in the air, and our eyes are much less sensitive to it. The mix we end up with looks sky blue.

Midday or sunset?

• Midday sky: Upside: short path, the sun stays bright and white, and scattered blue fills the sky. Downside: the colour is fairly pale near the horizon, where you look through more air.
• Sunset sky: Upside: deep oranges and reds, especially with a little dust or thin cloud to light up. Downside: much dimmer light, because so much has been scattered away.

Try it yourself: shine a torch through a glass of water with a few drops of milk in it. From the side the water looks slightly blue; looking straight into the torch through the glass, the light looks orange.