These rainbow days. Behind rain, the first rays of the sun often give us this beautiful sight painted in the sky. As a rule, we see only one rainbow, although it is not uncommon to see the so-called double rainbow.

This is the same gamut of colors, although displayed in a completely reverse order, somewhat lighter and much wider. Also, between one rainbow and another, there is a dark area known as Alexander group.

Just as in the case of a simple one, it is more normal to see a double rainbow early in the morning or at the end of the day, since that is when Sun rays they are lower. But beyond that, why do we sometimes see two instead of one?

Causes of a double rainbow

A primary rainbow is formed when water droplets left in the atmosphere after rain reflect light like a prismby dividing it into different wavelengths. These are the basics we already know. However, it can also happen that light passing through a drop of water is reflected twice, higher and lower.

Since the light is reflected twice, the reflections intersect and a second rainbow is formed in reverse order. That is, colors below the first can be seen above the second. Etc.

Another characteristic feature of a double rainbow is that the second rainbow, located above, Shire. This is because for this second reflection, the light, although dimmer, can be scattered over a wider region of the sky.

Alexander’s group

When we see a double rainbow, we also notice that between one and the other there is a very dark area, almost black. It is known as the Alexander group after its discoverer, the Greek philosopher Alexander of Aphrodisias.

This is because the light has already scattered up and down, so the water droplets between the two rainbows cannot be scattered towards the observer, i.e. us. As a result, since no light reaches us from there, we see a darker band. In other words, at this point, the light is absorbed and forms two arcs, so that remains in the main darkness.

It should be noted that although it is more common to see a simple rainbow, a double rainbow is quite common. However, sometimes the second one spreads faster across the sky and disappears before our eyes. You have to look at the right place, at the right time. It is worth remembering this the next time the sun rises low and after rain.

Source: Hiper Textual

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