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Sunsets are red. Clear skies are blue. Rayleigh Scattering makes it all be true. (Unless you are on Mars…)
Why is the sky blue?
When the Sun’s rays reach the Earth, they don’t just shine down uninterrupted on the planet’s surface. Thankfully, rays have to pass through the Earth’s atmosphere first. The air molecules scatter much of the light, sending it off in different directions in a phenomenon called Rayleigh Scattering.
But not all wavelengths of the Sun’s light are affected equally. The colors of light with longer wavelengths, like red, are not scattered as easily as those with shorter wavelengths, like blue and violet. The shorter the wavelength, the greater the intensity of the scatter, and the more of that color of light there is to see. Therefore, the visible light with the shortest wavelength – violent (indigo) – gets scattered the most.
So, the sky is… wait, what? Why isn’t the sky violet?
“Human eyes are more sensitive to blue light than violet. So, when a human looks up on Earth, the sky looks blue.
The intensity of the scatter (I) is inversely proportional to the wavelength (λ) raised to the fourth power. I = 1/λ
Aha! Great question! The Sun’s rays don’t contain the same concentration of all of the different colors of light. Even though the violet wavelengths get scattered more, the Sun emits a higher concentration of the blue light overall. Furthermore, human eyes are more sensitive to blue light than violet. So, when a human looks up on Earth, the sky looks blue.
With a hotter Sun, maybe 8000K, the higher intensity of violet light could probably make a purple sky. But the extra ultraviolet radiation would be too much for our sunscreen to handle.
Why are sunsets red?
At sunset on Earth, sunlight passes through the atmosphere at a much lower angle to reach the surface. By the time the light gets through the atmosphere, most of the blue light has already been scattered. What remains are the longer wavelengths, like red. That is why the sky appears red or orange at sunset on Earth.
What about the sky and atmosphere on Mars?
The Martian atmosphere is very thin, only about 1% of Earth’s. With so much less atmosphere to pass through, not as much scattering occurs. When there is no atmosphere at all, like on the Moon, light isn’t scattered at all and the sky appears dark.
Thanks to the low surface gravity on Mars, the air is often filled with fine dust particles from the ground. These dust particles are larger than the air molecules and contain iron oxides, providing the orange-red color that we often see on images from Mars. Those larger particles scatter light differently than Earth’s atmosphere, following a mechanism called Mie Scattering. This new kind of scattering happens when the particle size approaches the wavelength of the radiation.
While Rayleigh scattering scatters all light uniformly in all directions, Mie Scattering varies with scattering angle. Longer red wavelengths are scattered more uniformly while shorter blue wavelengths tend to be deflected. The Martian sky becomes reddish or yellowish-brown (butterscotch) during the daytime.
Just like on Earth, the Sun’s rays pass through the Martian atmosphere at a lower angle at sunset. The light passes through enough atmosphere that the light can be scattered by the smaller particles in the atmosphere with Rayleigh scattering. More blue light gets scattered and the sky near the Sun becomes blue during a Martian sunset.
If a little kid ever asks you just why the sky is blue, you look him or her right in the eye and say, “It’s because of quantum effects involving Rayleigh scattering combined with a lack of violet photon receptors in our retinae.”
— Philip C. Plait, Bad Astronomy