Question

why does the rainbow appear like semicircle​

Answer 1

The rainbow appear like semi circle because it is cause by refraction of light

Answer 2

Answer:

When making a rainbow, sunlight shining into each individual raindrop is refracted, or split into its component colors. And the light is also reflected, so that those various colors come bouncing back.

One key to rainbows is that the light leaves the collection of raindrops in front of you at an angle. In making a rainbow, the angle is between 40 and 42 degrees, depending on the color (wavelength) of the light.

The circle (or half-circle) results because there are a collection of suspended droplets in the atmosphere that are capable of concentrating the dispersed light at angles of deviation of 40-42 degrees relative to the original path of light from the sun.

These droplets actually form a circular arc, with each droplet within the arc dispersing light and reflecting it back towards the observer.