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Why is blue the sky?
by Ana 17 de Mayo de 2002



The sky, the earth's atmosphere, the privilege of the single living planet known. No matter where we are, we all share the same sky.

The colours in the sky vary from blue to white, pink, green, orange, violet… Its image is so changeful that you can not see it twice. The changing colour in the sky is due to the sunlight interaction with the planet atmosphere. Sunlight is white, also called polychrome (the addition of all the colours of the rainbow) and the atmosphere contains a certain amount of humidity, normally small, as well as dust and ash particles. The interaction between polychrome sunlight and atmosphere, with its particles hanging in the air, is enough to bring us a coloured sky.



Thinking about light going through a prism is an easy way to understand the reason why the sky is blue.

When a ray of light crosses over any material, its propagation direction deviates a certain angle, which depends on the type of this material. Transparent materials are usually characterized by a parameter called "refraction index", and its value depends on the colour of the light that comes across it. Therefore, each colour contained in a ray of white light will be deviated a different angle, bringing about a unique ray of white light separated into different colours behind the prism.

Each colour is characterized by a number called "wave length". The deviation of colours is maximum for the blue (with short wave length), that is, blue is the colour that diverts a wider angle from the initial light ray, and it is minimum for red and yellow (with a longer wave length), which do not diverges appreciably.

The blue light rays, once deviated from the initial ray, interact with other particles of the air, varying once more their direction. They make a zigzag trajectory through the atmosphere and finally they arrive to us. This is the reason because the rays seem to come from everywhere in the sky. Yellow rays of light do not actually deviates from the sun and, therefore, we can see the sun as being yellow.

When the position of the sun is near the horizon (low in the sky), its rays come across a higher thickness of air and will interact more often with the particles contained in it. The blue and violet rays are highly deviated to the sides than yellow and red ones, which maintain their trajectory in the same sunlight direction. This effect creates those wonderful sunsets that we can see from earth.


Rayleigh Dispersion

If we go deep in the phenomena related to the blue sky nature, we have to study the interaction between light and particles of small size, the so-called Rayleigh Dispersion.

Normally, when any electromagnetic wave (such as sunlight) finds in its trajectory an atom (or a small molecule), it interacts with its cloud of electrons, transmitting energy to the atom. This causes that the atom lowest level of energy (the natural state) begins to vibrate.

The oscillating electrons do not keep on vibrating for a long time. They return to the initial state by irradiating some of the energy they have got. Lord Rayleigh studied the dependence between the amount of irradiated light and the light wave length. He set up that the quantity of irradiated light is inversely proportional to the fourth power of the wave length. It can be represented as:

Therefore, if the incident light is white (an addition of different wave lengths or colours), the irradiated light will contain a higher amount of colours with shorter wave length (blue and violet) and less amount of colours with longer wave length (yellow and red). So we came out to the same conclusion than in the previous section.


Mie's Dispersion

Mie's Dispersion is used to study the interaction between light and particles of big size. We call a "big particle" when its size is bigger than a wave length. A big particle behaves like a mirror in the atmosphere, without any preference of any colour contained in the incident white light.

This type of interaction is the one that takes place between the sunlight and the clouds in the sky, since the clouds are composed by big sized non-coloured drops of water. These drops reflect as a mirror the white polychrome light that interacts with them, without changing the light colour. This is the reason because clouds appear white coloured in the sky.

When some particles of the air have a similar size of one wave length, of any colour, then they spread the light in a different way, irradiating a big range of different colours. This is the origin of the rest of the colours that we can see in the sky.


Bibliography and Questions


E. Hecht, A. Zajac, Optica, Ed. Addison-Wesley Iberoamericana, P. 253, 1990.

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