It is one of the first science questions many children ask, and a surprising number of adults are not sure of the answer. The explanation involves sunlight, the air around us and the way light interacts with very small particles.
Sunlight contains many colours
Sunlight looks white, but it is actually a mixture of colours. You can see this when light passes through a prism or when a rainbow forms after rain. Each colour corresponds to a different wavelength of light. Red light has longer wavelengths, while blue and violet light have shorter wavelengths.
Air scatters short wavelengths more strongly
Earth's atmosphere is made mostly of nitrogen and oxygen molecules. These molecules are much smaller than the wavelength of visible light. When sunlight passes through the air, the molecules scatter some of the light in different directions.
This process is called Rayleigh scattering, named after the British scientist Lord Rayleigh. Its key feature is that shorter wavelengths are scattered much more strongly than longer ones. Blue light is therefore scattered across the sky far more than red light.
When you look at any part of the sky away from the Sun, you are seeing this scattered light reaching your eyes from all directions. That is why the sky appears blue.
If violet is shorter, why is the sky not violet?
Violet light is scattered even more than blue, so this is a fair question. There are a few reasons:
- Sunlight contains less violet light than blue light.
- Some violet light is absorbed high in the atmosphere.
- Human eyes are more sensitive to blue than to violet.
Together, these factors make the sky look blue to us.
Why sunsets are orange and red
At sunrise and sunset, the Sun is low on the horizon. Its light must travel through much more atmosphere to reach you. Along this longer path, most of the blue light is scattered away before it arrives. The remaining light is dominated by longer wavelengths, giving sunsets their orange and red colours. Dust and other particles in the air can make these colours even stronger.
Quick summary
- Sunlight is a mix of colours with different wavelengths.
- Air molecules scatter short wavelengths, like blue, more than long ones, like red.
- Scattered blue light reaches our eyes from every direction, so the sky looks blue.
- At sunset, light travels further through the air, so reds and oranges remain.
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