The rainbow

The rainbow is a phenomenon that has always fascinated man, on one hand because generally it indicates that the end of a storm is near, on the other because of the sight offered by its colours. Who hasn’t been charmed by the display of a rainbow, perhaps against a sky darkened by threatening stormy clouds? About rainbows there are a number of myths and legends. For the Greeks it was a visible manifestation of a messenger of the Gods, according to Northern legends, at the far end of a rainbow is a pot containing a fabulous treasure.
In reality, a rainbow is a simple optical phenomenon, caused by the refraction of white light as it passes through the drops of water: analogously to the light that passes through a prism, the sun’s light that passes through a drop of water is refracted and separated into its different wavelengths. Diversely from the prism, through which we are able to see all the visible wavelengths of the spectrum at the same time, we are able to see one colour only, only one wavelength for each drop, depending on the height of the drop compared to our position, and the angle of light refraction from the drop to our eyes. Sunlight that simultaneously hits millions of microscopic drops of water is therefore refracted through each one of these (actually light is refracted twice, on entering the drop and when it passes outside). The result is that we see a series of concentric bands of coloured arcs drawn across the sky. The colours derive from the decomposition of the spectrum, that are visible at the different wavelengths, and are always in a precise order (depending on the wavelength) starting from the innermost arc with violet, indigo, blue, green, yellow, orange and red in the outermost arc.
A rainbow can be observed immediately after a downpour, when the sun breaks out between the clouds and it can be seen when the sun is behind our shoulders. A rainbow can form a complete arc from one point on the horizon to another, or only a part may be visible. At times double rainbows can also form, where one is always less brilliant and the sequence of colours is inverted, due to a complicated play of refraction and reflection inside and on the surface of the drops.
Iridescent halos and more faded rainbows with less brilliant colours may also form around the sun or the moon when these are veiled by cloud formations composed of ice needles, like the cirrus clouds. This type of rainbow can easily be seen in high mountain areas.

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