What is a rainbow?
#Author Donald Ahrens in his text Meteorology Today describes a rainbow as "one of the most spectacular light shows observed on earth". Indeed the traditional rainbow is sunlight spread out into its spectrum of colors and diverted to the eye of the observer by water droplets. The "bow" part of the word describes the fact that the rainbow is a group of nearly circular arcs of color all having a common center.
A rainbow isn’t really a “thing” and it doesn’t exist in a particular “place.” It is an optical phenomenon that appears when sunlight and atmospheric conditions are just right—and the viewer’s position is just right to see it.
Why Is a Rainbow appears Curved?
Technically, a rainbow is the upper half of a circle of light, which centers on the antisolar point, the point directly opposite the Sun, as seen from your perspective. The lower half of the circle, however, is usually not visible since the water droplets hit the ground before it can form. You may be able to see a circular rainbow if you have a high vantage point and the terrain sharply drops off in the direction of the rainbow, allowing the rain to fall down farther and reflect the sunlight from a lower angle. It is also possible to see a circular rainbow from an airplane.
The shape of a rainbow is a result of the refractive index of water. This causes the sunlight to be reflected off rain droplets within a limited range of angles that lie between 0° and 42°. Most of the light is cast back at you in an angular range from 40° for violet light to 42° for red light. This is why the circle of light always has an angular distance of 40-42° from the antisolar point, meaning a rainbow always appears 40-42° away from the point opposite the Sun, as seen from your perspective.
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