Rainbow flag Alliance (ICA) designed an international co-op symbol and a flag for the first "Co-operators' Day," which was held in July 1923. After some experiments with different designs, a famous French cooperator, Professor Charles Gide, suggested using the seven colours of the rainbow for the flag. He pointed out that the rainbow symbolized unity in diversity and the power of light, enlightenment and progress. The first co-op rainbow flag was completed in 1924 and was adopted as an official symbol of the international cooperative movement in 1925. In 2001, the ICA's official flag was changed from a rainbow flag to a
Rainbow 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. 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