A rainbow is most often viewed as a circular arc in the sky. An observer on the ground observes a half-circle of color with red being the color perceived on the outside or top of the bow. 

Those who are fortunate enough to have seen a rainbow from an airplane in the sky may know that a rainbow can actually be a complete circle. 

Observers on the ground only view the top half of the circle since the bottom half of the circular arc is prevented by the presence of the ground (and the rather obvious fact that suspended water droplets aren't present below ground). Yet observers in an airborne plane can often look both upward and downward to view the complete circular bow.

The circle (or half-circle) results because there are a collection of suspended droplets in the atmosphere that are capable 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. 

Every droplet within the arc is refracting and dispersing the entire visible light spectrum (ROYGBIV). 

As described above, the red light is refracted out of a droplet at steeper angles towards the ground than the blue light. 

Thus, when an observer sights at a steeper angle with respect to the ground, droplets of water within this line of sight are refracting the red light to the observer's eye. 

The blue light from these same droplets is directed at a less steep angle and is directed along a trajectory that passes over the observer's head. 

Thus, it is the red light that is seen when looking at the steeper angles relative to the ground. 

Similarly, when sighting at less steep angles, droplets of water within this line of sight are directing blue light to the observer's eye while the red light is directed downwards at a more steep angle towards the observer's feet. This discussion explains why it is the red light that is observed at the top and on the outer perimeter of a rainbow and the blue light that is observed on the bottom and the inner perimeter of the rainbow.

Rainbows are not limited to the dispersion of light by raindrops. The splashing of water at the base of a waterfall caused a mist of water in the air that often results in the formation of rainbows. A backyard water sprinkler is another common source of a rainbow. Bright sunlight, suspended droplets of water and the proper angle of sighting are the three necessary components for viewing one of nature's most splendid masterpieces.

Source: https://www.physicsclassroom.com/class/refrn/Lesson-4/Rainbow-Formation#:~:text=Rainbows%20are%20not%20limited%20to%20the%20dispersion%20of,that%20often%20results%20in%20the%20formation%20of%20rainbows.