Several readers have sent us photos of the same meteorological phenomenon, which could be observed on the coast around Boulogne-sur-Mer, but also in the hinterland, this Sunday, October 23. A double rainbow, how does it work? We take stock.
The sky of this Sunday, October 23 was of course, above all, marked by the terrible tornado which passed over the region and notably ravaged the village of Bihucourt.
But several readers have also sent us photos of a phenomenon that we could observe this Sunday at the end of the day in the Boulonnais: that of the double rainbow.
The rainbow, how does it work?
Technically, a rainbow is due to the prism effect of the refraction of the light ray in the drop of rainwater, which it passes through. In order for it to appear, it needs three ingredients: a shower, a sun halfway up the sky (no more than 41º) and the right position. Indeed, the sun must be placed behind the one who observes it, and the rain in front. Most of the time we observe a single rainbow, called primary arc, which forms on a circle whose center corresponds to the anti-solar point (a point opposite the Sun with respect to the observer).
A second arc still present
Around the main arc, called “primary” (the most colorful), it happens that a second less visible arc is seen. This is the “secondary” arc. This is caused by a double reflection of sunlight inside the raindrops. It appears in the opposite direction to the Sun, at an angle of 50 to 53º. This arch is in fact always present, but rarely visible.
Look at the pictures: the colors of this secondary arc are reversed compared to those of the first arc, with blue on the outside and red on the inside. Even less frequently, it happens that a third arc is visible, reversed in relation to the second and therefore identical to the first.
A darker area in between
Between the two arcs, there is a darker area called “Alexander’s band”. It is that after Aristotle explained the double rainbow phenomenon in 300 BC. JC, that Alexander of Aphrodisias, Greek philosopher, noted that the area between the two arcs is darker than the rest of the sky. The British physicist George Airy then showed that it was due to a phenomenon of interference between light rays.
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