The rainbow is formed by the refraction and reflection of the sun's rays as they fall on drops of rain which act as prism to produce the different colours.
The position of the sun,the rain drops and the observer affect the formation of the rainbow.
The rain drops which lie at this angle and in a direction opposite to the sun, lie in the form of a full circle or part of it(arc).Even if there are enough raindrops to form a full circle,it will look like an arc as it is limited by the horizon.
If you are in an aeroplane you can see a rainbow that forms a full circle!
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