Question

Why is rainbow is in bow shape?

Answer 1

due to refraction..............rainbow is in bow shape

Answer 2

It only involves sunlight, water, and your eye. So there's no reason that the Earth being flat or curved ought to change the shape of a rainbow. The reason a rainbowis circular is that a circle is the set of all points in the sky that make the same angle between you, the water droplets, and the sun.