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.
Why it cant be a square or any other polygon???
actually its a whole circle but we see a semicircle coz the water droplets are above us but if u sit an an aeroplane u can see the whole rainbow in the shape of a circle coz there the water droplets are present both above and below the plane