Everyone loves a clear sunny day, but have you ever looked up and wondered exactly why is the sky blue?
The answer lies in the physics of when sunlight passes through the atmosphere. The light rays are scattered in all directions as they hit the air molecules, and light at the blue end of the spectrum is scattered stronger than other colors.
Related: Earth's magnetic field: Explained
Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer.
In outer space, above the Earth's atmosphere, the sky looks black even in sunlight. That's one of the most striking phenomena experienced by travelers on Blue Origin's suborbital flights.
"To see the blue color go right by you and now you're staring into blackness, that's the thing," as William Shatner put it after his flight, trying to convey the experience he just went through. "The coloring of blue," he added.
This blackness is easy enough to understand. Unless you're looking directly at the sun, there's no reason for the sky to be illuminated at all. The real puzzle is why it's illuminated down here on the surface of the planet — a longstanding mystery that wasn't fully explained until the end of the 19th century.
The key lies in an effect known as Rayleigh scattering, after its discoverer Lord Rayleigh. This refers to the way light bounces off small particles, up to about a tenth the wavelength of the light itself — which includes the molecules making up the Earth’s atmosphere.
Rayleigh showed that longer wavelengths, corresponding to the red end of the spectrum, aren't scattered as strongly as short wavelengths like blue and violet. Of those two colors, it's the blue that dominates, partly because our eyes are more sensitive to it, and also because the sun emits less violet light to start with.
The sky isn't always blue. When the sun is low in the sky, at sunrise or sunset, it can take on a red hue. This is explained by the same physics — Rayleigh scattering — as the blueness of the sky at other times.
When we look towards the sun at sunset, we're seeing light that has traveled further through the atmosphere than when the sun is high in the sky. So most of the shorter wavelengths have been scattered away, and we just see what's left.
Additional resources
For more information about the physics behind the Earth's atmosphere and light, check out "The Earth's Atmosphere: Its Physics and Dynamics" by Kshudiram Saha and "A Ray of Light: A Book of Science and Wonder" by Walter Wick.
Bibliography
- NASA, "Why Is the Sky Blue?", accessed August 2022.
- Richard Luscombe, "William Shatner in tears after historic space flight: ‘I’m so filled with emotion’", The Guardian, October 2021
- C. R .Nave, "Rayleigh Scattering" Georgia State University, accessed August 2022.
- MET Office, "Why is the sky blue?", accessed August 2022.
- MET Office, "Why is the sunset red?", accessed August 2022.
- Royal Museums Greenwich, "What makes the sky blue?", accessed August 2022.
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