Why is the sky blue?
The short answer
Sunlight is a mix of every colour. Molecules in the air knock light aside, and the chance goes as one over the wavelength to the fourth power, so blue light at 450 nm is knocked aside about 6 times as often as red at 700 nm. That knocked-aside blue reaches your eyes from every part of the sky. At sunset the light crosses much more air, so most of its blue is knocked out on the way and what reaches you is red. Cloud droplets are far larger and scatter every colour alike, so clouds look white.
Chapters and transcript
- 0:00
Sunlight is white. So why is the sky blue?
Sunlight looks white, yet the sky around it is blue.
- 0:06
White light is every colour at once.
White sunlight is a mix of every colour. The film follows five of them, from red at 700 nm to blue at 450 nm.
- 0:12
In the air, molecules knock light aside.
Molecules in the air knock some of the light off its path. This is Rayleigh scattering.
- 0:19
Blue is knocked aside far more than red.
The chance of being knocked aside goes as one over the wavelength to the fourth power, so short blue waves are knocked aside far more than long red ones. The counts on screen come from the film's own photons.
- 0:26
About 6 times as often.
Blue at 450 nm is knocked aside (700 ÷ 450)⁴, about 5.9 times as often as red at 700 nm.
- 0:33
That blue flies off in every direction.
Knocked-aside light leaves the beam in every direction.
- 0:41
So it reaches your eye from all over the sky.
So light reaches you from every part of the sky, not only from the Sun, and most of it is blue.
- 0:48
That is the blue sky.
That knocked-aside blue light is the blue of the sky.
- 0:55
At sunset, the light crosses far more air.
When the Sun is low, its light crosses far more air on the way to you: 1 ÷ sin(elevation) times the thickness of the air, 8.2 times at 7° above the horizon in this flat picture.
- 1:03
The blue is knocked out on the way.
Along that long path, most of the blue is knocked out of the beam. In the film's example air, 13% of the blue is left and 70% of the red.
- 1:10
What reaches you is red.
What is left in the beam when it reaches you is mostly red and orange.
- 1:20
Cloud droplets are far bigger than molecules.
Clouds are made of water droplets, far larger than the molecules of the air.
- 1:28
They knock every colour aside alike: white.
Droplets that large scatter every colour almost equally, so clouds look white.
- 1:35
The blue sky is light knocked aside. The red sunset is what is left.
The blue sky is the light that air knocks aside; the red sunset is the light that is left after a long path through the air.
Sources
Every figure in the film comes from these. The animation is illustrative.