The Rainbow Connection

Check this out, then come back.

Theodoric (or Thierry, or Dietrich) of Freiberg (c.1250-c.1310) was a Dominican, a philosopher, and a physician. His name is often written with the title magister (master), so we know he had an advanced university education, almost certainly at Paris. In 1293 he was named Provincial of the Dominican Order, Albertus Magnus' old post.

Freiberg's description of the geometry of the rainbow.

We have 21 works written by him, although a list of works by Dominican authors compiled in 1330 lists 31 under his name. Somewhere between 1304 and 1310, Theodoric produced De iride et radialibus impressionibus (Concerning the rainbow and impressions of radiance). In it, he presents the correct explanation for the rainbow. He explains the primary rainbow, the secondary rainbow and why the colors are reversed, and the path light takes to make the rainbow.

That last is important, especially if you've read the link I gave you above and are aware of the competing theories for refraction and reflection, and the place of water droplets versus clouds. Freiberg accurately describes how the path of sunlight is refracted when it enters the droplet, reflected off the other side of the droplet, and refracted again when it leaves the droplet and becomes visible to the observer. Freiberg determined much of this by experimenting with glass spheres filled with water, an extraordinary act in itself in the history of scientific experimentation.

Perhaps, however, the mechanics of the rainbow was an idea whose time had come. In one of those examples of synchronicity that crop up in history from time to time, there was another scientist who came to some of the same conclusions as Frieberg. His name was Kamal al-Din al-Farisi, and he and Freiberg had no contact—although they did have one thing in common: they both knew the 11th century seven-volume work called The Book of Optics by Ibn al-Haytham. But that's for another day.