Why are rainbows curved
A rainbow is one the most beautiful natural phenomenon that is in existence and it is usually related closely to clouds and rainfall. One must note that a rainbow is not something that has any physiological existence; on the contrary, it is an optical effect of meteorological origin. This optical effect is seen when sunlight is reflected by the water drops present in the atmosphere (clouds and raindrops mainly). It is for this reason that a rainbow in the sky always appears on the opposite side of the sun.
The rainbow is famous for two of its attributes; first is its display of the basic colors (violet, indigo, blue, green, yellow, orange and red) and the second is the curved, bow like figure which it resembles. While the beautiful colors we see are the results of repeated reflection of sunlight after it enters water droplets present in air, the curved structure is not so easy to explain. There have been a few theories that have been propagated over the years, but none are conclusive enough to be established beyond doubt. We will now discuss one of the more popular theories in respect to this phenomenon.
In order to explain the phenomenon, one must at first understand the route of the rays of light that are responsible for the appearance of a rainbow. As soon as the ray of light (parallel to the light source or the sun) enters a drop of water, it gets refracted and then it is reflected off the other side of the water drop, internally. Then, just as the ray of light begins to leave the drop of water, it undergoes refraction once again. The deviation due to the entire process of refraction is found to be of an angle of 42 degrees (from the initial angle at which the ray entered the drop of water) when the ray of light finally emerges from the droplet.
Next, the concept of the antisolar point must be understood. The antisolar point is the point which is directly opposite to the position of the sun at that moment. During the peak hours of noon, one can determine the antisolar point by looking at their shadow ‘cause that’s where the head (of your shadow) will point. If someone looks at the sky at an angle of 42 degrees from the antisolar point, that person should be able to see the refracted rays of sun that has emerged from the droplets of water in the atmosphere. However, since the portion of the sky, which is 42 degrees away from the antisolar point, is not just a single direction but the infinite assortment of all the possible directions, it resembles the circumference of a circle around the antisolar point. This is the reason as to why a rainbow always looks curved, bow-shaped or semi-circular.
The theory explained above also states that though the height of the rainbow will mostly depend on the exact location of the sun when it’s above the horizon, the angle will never change and one will always see a rainbow appearing at a point in the sky which is at an angle of 42 degrees from the antisolar point.