Why Is the Sky Blue? And Why Are Sunsets Red?
The same sunlight makes both a blue noon sky and a red sunset. The secret is how air scatters different colours of light. Here is the simple explanation.
It is one of the first questions children ask, and one many adults cannot answer. The good news is that the explanation is simple and beautiful.
Sunlight is every colour at once
Sunlight looks white, but it is a mix of all the colours of the rainbow. A rainbow or a glass prism splits it into red, orange, yellow, green, blue, and violet. Each colour travels as a wave, and each has a different wavelength. Red has the longest wavelength, and blue and violet have the shortest.
What the air does to sunlight
Our atmosphere is mostly tiny molecules of nitrogen and oxygen. When sunlight meets them, the light bounces in all directions. This is called scattering. And the molecules are much better at scattering short wavelengths than long ones. Blue light is scattered far more than red light.
So when you look at any part of the sky away from the Sun, you are seeing blue light that has been bounced toward your eyes from all around.
"But violet scatters even more. Why isn't the sky violet?"
Good question. Three reasons work together:
- The Sun gives out less violet light than blue.
- Part of the violet is absorbed high in the atmosphere.
- Our eyes are far more sensitive to blue than to violet.
The mix we perceive is a pale blue.
Why sunsets are red and orange
At sunrise and sunset, the Sun is low, so its light has to travel through a much thicker layer of air to reach you. On this long path, most blue light is scattered away from your line of sight. What reaches you is the reds and oranges, the colours that scatter least.
Dust, smoke, and pollution can make sunsets even more red, because larger particles also influence the colours.
Try it at home
Fill a clear glass with water
Use a glass with straight sides.
Add a few drops of milk
Stir gently. The water should look slightly cloudy.
Shine a torch through it from the side
The liquid looks faintly blue from the side, like the sky.
Look at the light coming out the far end
It appears yellowish or orange, like a sunset.
What most people miss
- The sky is not blue because it reflects the sea. The sea is blue partly because it reflects the sky and partly because water absorbs red light.
- On the Moon the sky is black, because there is no air to scatter light.
- Mars has a butterscotch-coloured daytime sky and a bluish sunset, the reverse of Earth, because of its fine dust.
