Why is the sky blue?

On a clear day, we can see that the sky of the Earth has an intense blue colour. But, what is the cause for the sky to have this characteristic blue colour? To answer this question we need to know how light, and the molecules that form the atmosphere, interact between them.

The light

Light is a type of energy that travels in the form of waves. The smaller their wavelength is, that to say, the nearer a peak of a wave might be from the other, the higher its energy will be.

Each type of light is different from other ones due to its wavelength. The different types of light are classified in what we name the electromagnetic spectrum.

As we see, the visible light, also named white light, is composed of diverse colours, which go from purple to red. White light can de be decomposed using a prism. The spectrum of visible light can also be observed when a rainbow appears in the sky. Photons (which are the particles of light) that transport purple or blue light are the most energetic ones (within the framework of visible light), while the red ones, with a longer wavelength, are less energetic. When visible light contains all its colours, the resulting light is white (from here comes its name).

Interaction with the atmosphere

Light travels through space in a straight light, unless some object or celestial body obstructs its path. In this case, the object or body can affect the light in one of these three ways:

  • reflecting the light (like a mirror)
  • decomposing it (such as a prism)
  • or scatter it (as the molecules of the gases in the atmosphere)

When sunlight reaches the terrestrial atmosphere, it continues travelling in a straight line until it bumps into a dust particle or a gas molecule. What takes place at that moment depends on the wavelength of the light and of the size of the element it impacts against.

In the case that the visible light found itself with a dust particle or a water droplet, which have a much bigger size than the wavelength of visible light, this one is reflected in different directions. All colours of the light find themselves reflected by the particle in the same way. The reflected light appears white, as it contains all the colours of the light.

On the other hand, if light finds itself with a molecule of gas, what will take place will de different. The gas molecules are smaller than the wavelength of visible light, for which, when light hits one of these molecules, one part of it can be absorbed. After some time, the molecule radiates, that to say, releases, the light in another direction. The colour of the light that it releases is the same that the one that has been absorbed previously. The diverse colours of light are affected differently, even though all colours can be absorbed. But the molecules of the air absorb more frequently the photons of high energy (the blue ones), than the reds. This process is known as Rayleigh scattering, in honour of Lord Rayleigh, who was the first to describe this phenomenon, in 1871.

So, why is the sky blue?

The blue colour of the sky is due to the Rayleigh scattering. As light travels through the atmosphere, much light with short wavelengths is absorbed. The red, orange or yellow light is hardly affected by the air.

The shortest wavelengths (purple and blue) are the ones that are scattered the more intensely, reason for which there is much more blue light reaching our eyes.

We could also ask ourselves why the sky isn’t purple, as the light of this colour is scattered more than the blue one, as it is more energetic. This is due to that much of the purple light is absorbed in the high layers of the atmosphere. Moreover, our eyes are more sensitive to blue.

Why does the blue colour of the sky fade on the horizon?

As we lower our gaze to the horizon, we realise that the sky appears with a much paler colour, or even white. This is due to the fact that the light that arrives us from the horizon has had to travel more through the air, for which many of the blue colour light has been scattered many times in many directions. The surface of the Earth has also an important role in scattering and reflecting this light. Having the blue light being scattered so much, the dominance of this colour is reduced, and, so, we see higher levels of white light at the horizon.

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