The rainbow is a beautiful optical phenomenon that has fascinated humankind since antiquity. It is caused by a huge number of water droplets in the atmosphere illuminated by the Sun. Many noted physicists have contributed to the explanation of the rainbow. The wave theory of the rainbow was developed by George Airy, and modern descriptions are based on Mie scattering as developed by Gustav Mie. For educational purposes the simpler theory by Descartes, which is based solely on geometrical optics and is much more accessible, is used.