Did you know? Why the sky is blue and sunsets turn orange
Sunlight looks white, yet the sky turns blue and sunsets glow orange. Here is the science behind the colours of the sky, and why it isn't purple.
In this article
Every day we look up and see a blue sky. But sunlight actually looks white. So where does the blue come from, and why does the sky turn orange and red at sunset?
The answer lies in how light "bumps into" the air around us.
White light is really a mix of colours
Sunlight contains every colour of the rainbow. Each colour travels as a wave with a different length. Red has the longest wavelength, while blue and violet have the shortest.
When white light passes through a glass prism, these colours split apart and you can see each one clearly.
Air scatters blue light
Earth's atmosphere is mostly made of nitrogen and oxygen molecules. These molecules are far smaller than the wavelengths of visible light.
When sunlight hits molecules this small, the light is scattered in all directions. Shorter wavelengths, like blue, are scattered much more strongly than long wavelengths like red. This scattered blue light reaches our eyes from every part of the sky, so the whole sky looks blue.
This is called Rayleigh scattering, after the British physicist Lord Rayleigh (John William Strutt), who explained it.
Did you know? The sky near the horizon looks paler than the sky overhead. Light from that direction passes through more air and is scattered again and again, so the colours mix and turn whiter.
If violet is shorter, why isn't the sky purple?
Good question! Violet is actually scattered even more strongly than blue. But the sky still looks blue for a few reasons:
- Sunlight isn't evenly balanced. The light arriving from the sun contains more blue than violet.
- Violet is lost earlier. As light travels through the atmosphere, much of the violet is scattered away before it gets close to us.
- Our eyes are more sensitive to blue. Human eyes detect colour with three types of cells, and our brain reads the mix of scattered light as blue.
Why do sunsets turn orange and red?
At sunrise and sunset, the sun sits low on the horizon. Its light has to cross a much thicker layer of atmosphere before it reaches us.
Along that long path, almost all the blue light is scattered away. What remains are the longer wavelengths: yellow, orange and red. That is what gives sunsets their beautiful colours.
Interestingly, the same process that makes the sky blue during the day also makes it red at sunset. The only difference is how far the light travels.
Did you know? On Mars, it's the other way round. The daytime sky there is a yellowish brown because of dust, while sunsets have a bluish glow around the sun.
In short
- Sunlight is a mix of all colours.
- Air molecules scatter blue light much more strongly than red, so the sky looks blue.
- The sky isn't purple because the sun gives off less violet light and our eyes are more sensitive to blue.
- At sunset, light passes through more air, the blue is scattered away, and only orange and red remain.
Next time you look up at the sky, you'll know its secret!
