Science & Nature - Rainbows: the atmospheric physics of multicoloured sky arcs

A rainbow is an optical phenomenon created when sunlight undergoes refraction, internal reflection, and dispersion inside airborne water droplets. This process splits white light into a spectrum, always appearing directly opposite the Sun. While ground observers usually see an arc, rainbows viewed from aircraft can form complete circles. A primary arc shows red on the outside and violet inside, whereas a secondary rainbow appears ten degrees farther out with reversed colors. The longest-lasting rainbow on record endured for nearly nine hours over Taiwan in 2017.

Source: Wikipedia