<P> Descartes' 1637 treatise, Discourse on Method, further advanced this explanation . Knowing that the size of raindrops did not appear to affect the observed rainbow, he experimented with passing rays of light through a large glass sphere filled with water . By measuring the angles that the rays emerged, he concluded that the primary bow was caused by a single internal reflection inside the raindrop and that a secondary bow could be caused by two internal reflections . He supported this conclusion with a derivation of the law of refraction (subsequently to, but independently of, Snell) and correctly calculated the angles for both bows . His explanation of the colours, however, was based on a mechanical version of the traditional theory that colours were produced by a modification of white light . </P> <P> Isaac Newton demonstrated that white light was composed of the light of all the colours of the rainbow, which a glass prism could separate into the full spectrum of colours, rejecting the theory that the colours were produced by a modification of white light . He also showed that red light is refracted less than blue light, which led to the first scientific explanation of the major features of the rainbow . Newton's corpuscular theory of light was unable to explain supernumerary rainbows, and a satisfactory explanation was not found until Thomas Young realised that light behaves as a wave under certain conditions, and can interfere with itself . </P> <P> Young's work was refined in the 1820s by George Biddell Airy, who explained the dependence of the strength of the colours of the rainbow on the size of the water droplets . Modern physical descriptions of the rainbow are based on Mie scattering, work published by Gustav Mie in 1908 . Advances in computational methods and optical theory continue to lead to a fuller understanding of rainbows . For example, Nussenzveig provides a modern overview . </P> <P> Experiments on the rainbow phenomenon using artificial raindrops, i.e. water - filled spherical flasks, go back at least to Theodoric of Freiberg in the 14th century . Later, also Descartes studied the phenomenon using a Florence flask . A flask experiment known as Florence's rainbow is still often used today as an imposing and intuitively accessible demonstration experiment of the rainbow phenomenon . It consists in illuminating (with parallel white light) a water - filled spherical flask through a hole in a screen . A rainbow will then appear thrown back / projected on the screen, provided the screen is large enough . Due to the finite wall thickness and the macroscopic character of the artificial raindrop, several subtle differences exist as compared to the natural phenomenon, including slightly changed rainbow angles and a splitting of the rainbow orders . </P>

Colors of a rainbow can be seen because