Question

why the rainbow is formed in an arc???

Answer 1

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 rainbow is 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.

Answer 2

Rainbows: an arc of concentric colored bands. A rainbow is an arc of concentric colored bands that develops when sunlight interacts with rain drops. A rainbow occurs when rain is falling in one portion of the sky and the sun is shining in another.