The Snell-Descartes reflection law (1621) allows to accurately modelize the path of a monochromic light through a water droplet. The colours of the rainbow were only explained by Newton through his prism experiment in 1666. The supernumerary arcs were first qualitatively explained by Young in 1803, by showing that light is capable of interferences (see Figure). The treatment of diffraction was quantitavely studied by Airy in 1848 and the treatment of polarization by Brewster in 1815, but a "complete" theory of the rainbow did not emerge before the beginning with the French Mascart in 1912, the last one of a long series of scientific papers establishing the so-called standard "Airy's theory" of the rainbow.