The color of the sky

An orange sunset below a blue sky

Where does the blue color of the sky come from?

The blue color of the sky is something we take for granted, but how does it happen? Before it enters the atmosphere, the light from the sun contains all the wavelengths in the visible spectrum, and the sun would appear white to an observer in space. As the sunlight travels through the atmosphere, the spectrum gets modified. Ozone, water vapor, and other gases absorb some wavelengths. The light also gets scattered by molecules, clouds, and larger particles (referred to as aerosols). The blue sky on a clear day is caused by scattering from the most abundant molecules, oxygen O2 and nitrogen N2.

Scattering

Scattering can be described by analogy to waves in a water tank encountering a thin cylinder protuding from the water's surface. Here's an animation of the process looking down at the water's surface.

Waves come in from the left, a thin cylinder intercepts some of the energy, producing a circular scattered wave

A wave comes in from the left, and the cylinder scatters a small fraction of the energy in all directions. The resulting scattered wave can be seen as a circular wave emanating from the cylinder.

Light consists of waves of oscillating electric and magnetic fields. When a light wave encounters a molecule, its electric field tugs the molecule's electrons back and forth. The resulting back and forth flow of charge is exactly what happens in a transmitting antenna. It causes the molecule to give off an electromagnetic wave that emanates in almost all directions. This is the scattered wave. Here's an animation of the electric field pattern of this wave.

Electromagnetic waves produced by an up-and-down movement of charge

So why blue?

It turns out that high frequency (or equivalently, short wavelength) light is scattered much more strongly than low frequency (or equivalently, long wavelength) light. The amount of energy scattered increases as the fourth power of the frequency. This means that blue light is scattered much more strongly than red light.

Blue light is strongly scattered out of the sunray, leaving light that is "reddened"

Now we can see the consequences for two observers, shown above. Observer A is looking up at the sky and the sun ray is crossing his field of view. Blue light is preferentially scattered into his line of sight. Observer B is watching a sunset. By the time the sun ray reaches her, it has been depleted of blue light, giving it a reddish hue.

To learn more

I've described the scattering of light by objects much smaller than the light's wavelength. This is called Rayleigh scattering, after the British physicist Lord Rayleigh (1842-1919). You can read more on this Wikipedia page.