Newton's rainbow explanation. Diagrams explaining the optics of a rainbow, by the English physicist, mathematician and astronomer Sir Isaac Newton (1643-1727). The diagrams were published in his famous work Opticks (1704). The main diagram shows the refraction and reflection of sunlight (S) inside raindrops (circles). In the main rainbow (bottom, E and F) the rays are refracted twice and reflected once. In the secondary rainbow (upper, G and H) the rays are refracted twice and reflected twice. This is also shown in the surrounding diagrams. This causes the secondary rainbow to be both fainter, and have its colours reversed. This explanation dates back before Newton to Theodoric of Freiberg in 1307.