Some rainbows appear in double because sunlight is reflected twice inside raindrops, creating a first main rainbow and a second, weaker one located above.
A rainbow forms due to the reflection and refraction of light in water droplets suspended in the air. When light enters a droplet, its colors separate because they travel at different speeds in water. A double rainbow occurs when light bounces twice within the droplets before emerging. This second bounce creates a second arc, which is always fainter. A nice detail to observe: the second arc (secondary arc) always displays colors that are reversed compared to the first one (primary arc).
When a ray of light enters a drop of water, it is reflected and refracted. A single bounce produces the primary arc, but sometimes, light can make a second bounce inside the drop before exiting. This slightly more complicated path is essential for forming a secondary arc. This second bounce also explains why the secondary arc always appears fainter and inverted, with the colors in the reverse order compared to the primary arc. Without this double reflection, there is no second arc. Simple and effective.
The first arc, known as the primary arc, is always the brightest, with vibrant colors, featuring red on the outside and violet on the inside. Just above it, there is often a second, less luminous arc called the secondary arc, where the colors are reversed: this time, red is on the inside and violet on the outside. This second arc is not only paler, but it also extends over a slightly wider area in the sky. Between the two, one can notice a darker band called the Alexander's band, where the brightness is significantly lower. These features arise from the fact that the secondary arc results from a double reflection of light inside the raindrops, which explains why it is a bit less intense and why its colors are inverted.
To hope to see a double rainbow, you first need a precise combination of light rain and a cleared sky. Generally, this often happens after a downpour, when the sun suddenly reappears behind you. The more abundant and uniform the raindrops are, the greater your chances of clearly observing a second arc, even if it is always a bit less bright. On the other hand, if the rain is too dense or too light, the effect will be significantly less visible, or even nonexistent. Finally, sufficiently intense and clear sunlight enhances the colors and sharpness of the secondary arcs.
A strong brightness clearly enhances the visibility of the secondary rainbow: basically, the sunnier it is, the more likely the secondary arc is to be visible. The viewing angle is equally important: always prefer to have the sun at your back, generally at a low angle on the horizon. If you change your position, the rainbow moves too, as it depends on your viewpoint: you always see your arc at about 42 degrees for the primary and about 51 degrees for the secondary, starting from the imaginary axis passing through your head and the center of the arc. If you observe from a plane or a mountain, the rainbow can even become circular, which is pretty nice to see!
Did you know that all rainbows are actually complete circles, not just semi-circles? From the ground, our horizon limits our view to a half-arc, but from an airplane or a high mountain, one can exceptionally observe a complete circular rainbow.
Each observer perceives their own rainbow, depending precisely on their location. Thus, even if two people stand side by side, they do not see the same rainbow, but rather two slightly different rainbows according to their individual angle of view.
The maximum number of colors present in a rainbow varies across cultures. Traditionally, in the West, it is considered to have seven colors, but in other parts of the world, such as Japan, the rainbow is sometimes described as having only five colors.
Although rare, some observers have reported seeing a third or even a fourth rainbow. Referred to as tertiary and quaternary rainbows, these phenomena are extremely faint and are often found in the direction opposite to the Sun, making them much more difficult to observe with the naked eye.
The best angle of observation is with your back to the sun and the rain in front of you. Generally, an angle of about 42° for the primary rainbow and 51° for the secondary rainbow optimizes their visibility.
Rainbows form when water droplets suspended in the air refract, reflect, and disperse sunlight. This is why they are often seen after rain, when the sun begins to reappear through the clouds.
Yes, this phenomenon is called a moonbow. It appears when the light reflected by the moon is sufficiently intense and the weather conditions are favorable, but it is less vividly colored than that produced by the sun.
Yes, triple or quadruple rainbows do exist but are rare and very difficult to observe. They result from multiple light reflections inside the droplets, significantly reducing the perceived light intensity.
The second rainbow appears due to a double reflection inside the water droplets, resulting in a complete inversion of colors compared to the primary rainbow.
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