Rainbows are colored arcs seen when falling water drops refract and reflect the Sun's light. The primary bow with its center at the antisolar point is the brightest. It has a radius of 42°. Very often there is an outer secondary bow visibly with a radius of 51°. A rainbow can only be seen from the ground when the sun is less than 42° above the horizon. Therefore rainbows are seen early in the morning and late in the afternoon. They often occur after thunderstorms or rain showers in the summer time.

The rainbow is formed by droplets at least 0.05 mm in diameter. The size of the droplets is the most important factor for the intensity of the rainbow. Bigger drops give brighter rainbows.

The area between the primary and secondary bow is darker then the surroundings and is called Alexander's dark band after a Greek who noticed it over eighteen hundred years ago.

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