Full rainbows can form when sunlight is refracted, reflected, and dispersed inside raindrops in such a way that it returns to the observer. This creates a complete circle of colors.
When sunlight passes through a droplet of water, it is both refracted, reflected off the inner surface, and then refracted again as it exits. These multiple changes in direction split white light into different colors. Each droplet acts like a mini prism sending a colored beam back to the observer. The complete circular shape comes from the fact that these colored rays exit at a precise angle, about 42 degrees, creating a light cone with the observer's eye always at the apex. The rainbow actually forms a whole circle, but most of the time, the horizon limits the view, reducing it to a simple arc.
In fact, the angle from which you observe a rainbow completely changes its appearance. A classic rainbow actually always forms a whole circle, but from the ground, you only see a part of the circle because the landscape blocks the rest. This precise angle, about 42 degrees, directly depends on the position of the sun behind us in relation to the water droplets in front. As soon as you move, this circle "follows" you because the rainbow is not fixed in space: it is only related to your position of observation and the trajectory of light rays in the droplets. Therefore, to see the complete circle, you need to be high enough to clear any obstruction and have the sun ideally placed behind you.
From the ground, it is almost impossible to observe a complete rainbow. Why? Because the ground itself blocks the lower part of the luminous circle. As one gains altitude, for example in a plane or on a mountain, the field of vision significantly widens. As a result, the horizon descends below the observer's feet, freeing the view of the usually hidden lower part of the rainbow. If one is high enough with the sun behind them, they can then admire a magnificent unbroken colored circle, and that is quite spectacular.
To see a circular rainbow, one must be positioned high enough to have an unobstructed view downward, such as from an airplane, in the mountains, or from a tall tower. It is essential to have the sun at your back and to face the droplets of water suspended in the air in front of you. The ideal angle between the sun, your eye, and the water droplets is about 42 degrees. The higher you are, the greater your chances: no obstacles will then obstruct the view of this complete round shape. The ambient light must be balanced, with a partly cloudy sky and brief, light showers. These droplets must be numerous, large enough, and evenly distributed to clearly create the optical phenomenon.
Did you know that the angle required to observe a rainbow is precisely 42 degrees between your position, the raindrops, and the direction opposite to the sun? This is why rainbows are only visible when the sun is relatively low on the horizon.
Did you know that the majority of secondary rainbows (double rainbows) have their colors inverted compared to the primary rainbow? The violet is on the outside, while the red is on the inside.
Did you know that each observer sees their own rainbow? Due to the precise angle of the light rays reflected in the raindrops, two observers located in different places never see exactly the same rainbow.
Did you know that there is a phenomenon called a 'lunar rainbow,' formed by light reflected from the Moon rather than from the Sun? These rainbows are very faint and hard to observe due to the low brightness of moonlight.
Rainbows are more easily observed when the sun is low in the sky, usually early in the morning or late in the afternoon. At these times, the angle of sunlight allows for optimal reflection inside the water droplets.
Yes, nighttime rainbows, known as lunar rainbows or moonbows, do exist. They are much fainter and harder to observe. The light reflected by the moon replaces that of the sun, which explains why their visibility is reduced and their occurrence is rare.
The colors of the rainbow result from the refraction and the specific angles at which different wavelengths of light emerge from water droplets suspended in the air. The color red, with the longest wavelength, appears at the top outside of the arc, while violet, having the shortest wavelength, appears at the bottom inside the arc, thus creating a constant order of colors.
A double rainbow occurs when sunlight undergoes double internal reflection within water droplets. This creates a secondary arc with reversed colors, which is fainter and located above the primary rainbow.
It is extremely rare to see a complete circular rainbow from the ground due to the observer's position on Earth. Generally, the horizon obscures part of the circle, and only a half-rainbow is visible. However, from a high altitude, such as during a flight or from a mountain peak, it is possible to see the entire arc.
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