Question

why rainbow is formed in arc only why not every shape formed while rain drop is available everywhere ?

Answer 1

The reason why the rainbow is curved is because all the angles in the water drop have to be just right for the drop to send some sunlight to you, standing on the ground. So, with the sun *behind* you, only those water droplets that have the same angle formed by you, the drop, and the sun (this angle happens to be approximately 42 degrees) will contribute to the rainbow. Other droplets send their light somewhere else.
Only when every angles are same when it's an arc.If it a straingt line,as we look to left or to right,the angle will increase more than that made with the centre .