Question

why rainbow is hemispherical

Answer 1

The rainbow is circular because when a raindrop bends light, the light exits the raindrop at an angle 40 to 42 degrees away from the angle it entered the raindrop. and regarding full circular rainbow, But if the sun is very low in the sky, either just before sunset or just after sunrise, we can see a half circle."

Answer 2

Sometimes the rainbow shows up after rain with stormy weather. From the ground it may seem like a semi-circle,
                    but a pilot flying in the sky would often see a full circle. The reason rainbows don't look like a full circle to us is because we are too low to the ground to see the rest.


The subtle droplets of water hovering in the air after rain play the role of a crystal. Passing through the droplets of water/moisture particles sun rays are bended,
hitting the internal surface of the moisture particle they are reflected outward and again get a bend.
       This is the principle of a crystal. Hence they diversify into seven colors.