Optics · a Light

He built a glass raindrop to explain the rainbow

Around 1309 in Tabriz, Kamal al-Din al-Farisi was working through Ibn al-Haytham's Optics at the suggestion of his teacher Qutb al-Din al-Shirazi, and reached the rainbow. Rather than argue about it he built a model: a large clear glass sphere filled with water, standing in for one drop, set up in a darkened room. Tracing the light through it, he found that the main bow comes from two refractions and one reflection inside each drop, and the fainter outer bow from an extra reflection, which is also why its colours run the other way. At almost the same time Theodoric of Freiberg in Germany reached the same conclusion by the same kind of experiment. There is no sign either knew of the other.

Kamal al-Din al-Farisi · Tabriz, c. 1309