why the rainbow is in semi circle? why cant be a circle?

 it is a full circle, because the perfectly spherical water droplets in the air split the white sunlight into the spectrum which you see as a rainbow. The light comes in at an angle, and then reflects out at an angle. From your perspective on the ground, that angle exsists in a full 360 degree arc - a circle. You only see a semi-circle because either the earth gets in the way of seeing the rest, or because there isn't sufficient water suspended in the atmosphere to see the whole thing.