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Section 2 - Refraction


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.
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