A rainbow is produced as a result of refraction, reflection, and so colour dispersion of sunrays in millions of water drops. The drops are circular rather than dropshaped, as you might have thought. The rainbow appears as a shining circle in the sky, when we observe falling rain with the Sun behind us. The observer’s line of sight to a random point on the rainbow produces a 42 degree angle with the direction of the sunlight.
The observer only sees the light reflected at a specific angle from the raindrops.