Cobra Effect · Light and electricity
Why the sky is blue
How a hazy glass tube and a calculation explained the colour of the sky.
7 cards, read aloud in 3:15, with a test and sources.
Sunlight is white, yet the sky around the Sun is blue.
Newton showed that white sunlight is a mixture of every colour. So something in the air must be picking out the blue part and sending it towards us from every direction. For a long time nobody could say what that something was. The answer came in steps: first an experiment, then a calculation.
In 1869 John Tyndall made a blue sky in a glass tube.
At the Royal Institution in London, he shone a beam of white light along a glass tube and let a fine haze form inside it. Seen from the side, the beam glowed blue. The light that carried straight on through was left a little redder, because blue had been sent off sideways. Tiny particles, it seemed, scatter blue light more than red.
In 1871 Lord Rayleigh worked out how strongly each colour is scattered.
He showed that very small particles scatter short waves far more than long ones. Halve the wavelength, and the scattering goes up sixteen times. Blue light has a shorter wavelength than red, so it is scattered more than five times as strongly. That is enough to turn scattered sunlight blue.
Later, Rayleigh showed that the molecules of the air are enough on their own.
Tyndall had put the blue down to fine particles floating in the air. By 1899 Rayleigh had shown that the molecules of nitrogen and oxygen scatter enough light by themselves. So the sky would be blue even if the air were perfectly clean. Clouds are white for a different reason: their droplets are much bigger, and scatter every colour about equally.
At sunset, the light has much further to travel through the air.
When the Sun is high, its light crosses the air by a short path. At sunset it comes in low, through far more air on its way to your eyes. Along the way, most of the blue is scattered off to the side, leaving the reds and oranges. The same scattering that makes the sky blue makes the sunset red.
Violet is scattered even more than blue, so why is the sky not violet?
Sunlight holds less violet than blue to begin with, and some violet is absorbed high in the atmosphere. Our eyes are also much less sensitive to violet than to blue. Put together, the scattered light looks sky blue to us. The colour of the sky depends on the Sun, the air, and the eye that looks at it.
So when an answer seems obvious, ask whether it would still hold somewhere else.
On Mars, fine dust hangs in the thin air. The daytime sky there is a dusty butterscotch colour, and around the setting Sun it turns blue. That is the reverse of Earth, because dust grains that size scatter light differently from air molecules. The rules of scattering are the same everywhere. What changes is what the light meets.
Sources
- John Tyndall’s blue sky apparatus, Royal Institution. The tube Tyndall used in 1869 to make a small blue sky.
- Rayleigh scattering, Wikipedia. Why small particles and molecules scatter short waves far more than long ones.
- Why is the sky blue?, Atmospheric Optics. A careful account of the colour of the sky, including why it is not violet.
Nearby ideas
- White light is every colour at once. How a second prism showed the colours were in sunlight all along.
- There is light on both sides of the rainbow. How thermometers, silver salts and a glowing screen found light we cannot see.
- Light takes time to arrive. How a moon of Jupiter and a spinning wheel showed that light takes time to arrive.
- Lightning is a giant spark. How a thread, a jar and an iron rod showed that lightning is electricity.
- A frog’s twitch led to the first battery. How an argument over twitching frogs’ legs gave the world its first battery.
- A moving magnet makes electricity. How a swinging compass needle led, in eleven years, to the first generator.
- Light is a wave of electricity and magnetism. How equations predicted invisible waves, and sparks across a room proved them real.