This is how rainbow works
Rainbows aren’t just arches; they are full circles. We usually see only the bottom half because we observe them from ground level. Every rainbow is a circle in theory, but the horizon hides the rest from view.
Years ago, workers at the Lakhta Center in Saint Petersburg captured a striking example of a complete circle rainbow. This 87-story tower soars to 462 meters, making it the tallest skyscraper in Europe and among the world’s tallest. Such height provides a rare perspective for contemplating a full rainbow.
To glimpse a complete rainbow, you must be at a considerable altitude with water droplets below the visible horizon. That combination is uncommon for most observers on the ground.
The rainbow’s center lies opposite the Sun. As the sun moves toward the horizon, more rainbows can be seen, and the largest visible portion—about half of the circle—tends to occur around sunrise or sunset.
This is how rainbow works
Ángel Rodríguez, writing for Cienciaes, describes the rainbow as a natural interplay among three players: sunlight, raindrops, and the human eye. The observer’s perception is unique because each eye views a slightly different arc. With both eyes close together, the brain blends the images, producing a single rainbow for each viewer.
The classic explanation holds that sunlight refracts inside tiny water droplets, splits into colors, and exits the droplet in multiple directions. When the observer looks in the direction opposite the Sun, they see the spectrum formed by these refractions and reflections.
Better than airplane
Regarding viewing a full circle rainbow, the idea is straightforward: from a high altitude above clouds, the line connecting the Sun to the observer and the rainbow’s center can be clear of obstacles. If enough droplets are present, nature can reveal a full circumference of color.
One might wonder if a mountain summit provides the same effect. It often does not, because the mountain can block the Sun’s rays and close the rainbow’s circle with its shadow. The solution is to observe from a high-altitude vantage point—whether from an airplane, a helicopter, or a skyscraper as tall as the Saint Petersburg tower. In such cases, the observer and any vehicle can cast a shadow at the circle’s center without interrupting the rainbow’s spread, provided the shadow remains small compared to the radius of the arc.
In summary, the full circular rainbow remains a rare and breathtaking phenomenon best appreciated from elevated vantage points where water droplets are plentiful and the Sun is positioned favorably.
Source note: ideas summarized from Cienciaes and observational explanations about circunferencia iris. Cited from Cienciaes.
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