Rainbows are always rounded because they are formed by the refraction and reflection of sunlight through water droplets suspended in the air, creating a circle-shaped spectrum of colors.
When sunlight enters a droplet of water, its path changes slightly: this is called refraction. In the process, white light splits into several distinct colors (red, orange, yellow, green, blue, indigo, and violet). This phenomenon is called light dispersion. Each color bends at a different angle because they do not all have the same wavelength: red is the least deviated, while violet is the most. It is this decomposition that creates the characteristic color range of the rainbow.
When a ray of light enters a droplet of water, part of it exits directly, but another part is reflected inside. From a certain angle, the ray no longer exits at all and bounces entirely within the droplet: this is total internal reflection. This internal reflection acts like a natural mirror and sends the ray back to our eyes. Without it, there simply would be no rainbow because the light would just pass through the droplets without returning to you. It is thanks to this total internal reflection that the colors come out so clearly and form this beautiful luminous arch.
In reality, there is not just one single rainbow: each observer sees their own arc, depending on their angle of view. This angle is always about 42 degrees relative to the direction opposite the sun. When you move, the rainbow moves with you, because each drop of water projects its rays of light at this same precise angle towards your eyes. No one sees exactly the same rainbow, as each person positions themselves differently in relation to the drops and the sun. That is why it is impossible to approach a rainbow or to go underneath it; it always remains at the same distance from you, true to your personal angle of view.
Every raindrop naturally has an almost spherical shape due to surface tension. It's like a small floating bubble, perfectly round, that refracts and reflects light in all directions at once. This shape causes each drop to act a bit like a tiny spherical mirror and a transparent lens. The result: when sunlight enters a droplet, it follows a precise path, bending at specific angles. It is this rounded geometry that ultimately allows for the appearance of a beautiful rainbow with soft and harmonious contours rather than straight or irregular shapes.
Your position of observation and that of the sun define a precise axis around which the rainbow forms. Each droplet of water reflects and refracts light at a precise angle of about 42 degrees, creating a light cone centered on the imaginary line connecting the sun, your head, and the axis opposite the sun. Since your eye is located exactly at the top of this cone, you naturally see a perfectly circular arc; at least, a part of it (the ground cuts off the rest of the circle!). Moreover, if you were in an airplane or on a high mountain with the sun low behind you, you could see the entirety of a rainbow, fully circular. So, it’s not just an artistic shape of nature, but a logical – and geometric – consequence of simple rules of optics.
Did you know that the colors of a rainbow always form the same sequence: red, orange, yellow, green, blue, indigo, violet? This sequence depends on the wavelength of visible light.
Did you know that each person sees their own personal rainbow? Since each observer is in a slightly different position, everyone will see the light refracted by different raindrops, making their experience unique.
Did you know that it is possible to observe a double rainbow, with the second arc displaying the colors in reverse? This is due to a double reflection of light within the water droplets.
Did you know that some rare rainbows, called 'white rainbows' or 'fog bows,' appear white because the water droplets that create them are extremely fine, thus limiting the dispersion of colors in the light?
A double rainbow occurs when light undergoes double internal reflection inside water droplets before emerging. The second arc, which is positioned outside the first, is fainter due to a greater loss of light intensity and displays its colors in reverse order (with red on the inside and violet on the outside).
The colors always appear in the same order (red on the outside, violet on the inside) because each color is refracted at a slightly different angle as it passes through the water droplets. Red is the least refracted and violet is the most refracted, resulting in this regular order in the visible spectrum.
Yes, this is called a moonbow. This phenomenon, relatively rare but entirely possible, is caused by the reflection, refraction, and dispersion of moonlight through water droplets. However, the colors of a moonbow are generally paler and sometimes even nearly white due to the low light intensity.
A rainbow is not a physical object located in a specific place, but an optical phenomenon that depends on the observer's position and their angle of view relative to the water droplets. As you move, the rainbow seems to move as well, making it impossible to truly reach its ends.
From the ground, an observer can generally see only a circular portion of the rainbow. However, from an airplane or a mountain, it is possible to see a complete rainbow forming a full circle if the conditions are favorable.
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