2 min
Why is the sky blue and not purple?
Answer
At noon the sky is blue. At sunset it is still blue. But sunlight is white, and white is every colour at once.
What you are looking at is not a blue sheet. It is a whole layer of air, glowing.
Take a guess first: is the sky painted blue by the sun, or filtered blue by the air?
1. Let the molecules wobble
Sunlight hits the air and shakes the electrons inside each molecule back and forth. A wobbling molecule turns into a tiny antenna and sends out new light in every direction.
2. Blue gets thrown the farthest
The shorter the wavelength, the harder the shake, and the more light gets thrown. Blue is flung about four to six times as much as red — so the whole sky keeps sending blue into your eyes.
3. Your eyes drop the violet
Violet is thrown even harder. But the sun gives us less of it, ozone eats some, and our eyes are barely sensitive to it — so the brain reads the mix as blue.
Guess with me
- Guess: if the air suddenly became ten times thicker, would the daytime sky turn deep blue, or wash out pale?
- Try: just before sunset, compare the colour overhead with the colour beside the sun. Tell me tomorrow how different they were.
- Ask: some people say the sky is blue because the sea reflects blue upward. What is wrong with that?
Send your answer to Michael
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I'm Michael
I stared at the sky for a while. It stopped looking like a colour and started looking like light taking the long way round.
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