Theodoric of Freiberg on rainbow phenomenon

The rainbow is a bridge between the two cultures: poets and scientists alike have long been challenged to describe it. The ancient Greek philosophers tried to describe the rainbow, and Aristotle was the first to fully include it among the phenomena studied by physicists.

He proposed that the rainbow is actually an unusual kind of reflection of sunlight from clouds. The light is reflected at a fixed angle. giving rise to a circular cone of "rainbow rays. " Aristotle thus explained correctly the circular shape of the bow and perceived that it is not a material object with a definite location in the sky but rather a set of directions along which light is strongly scattered into the eyes of the observer.

Al-Hazen suggested that, in order to be able to form a rainbow, sunlight had to be reflected by clouds before reaching human eyes. Thus the water drops composing the clouds reflect the light ray and create the rainbow’s colours through a refraction and two or more reflections, like an image forming in a reflecting, smooth, concave, spherical mirror consisting of dense, wet air.

In 1304, Theodoric of Freiberg, the German monk, suggested that the rainbow phenomenon is due to sunlight being reflected through water drops suspended in mid-air.

He suggested instead that each drop is individually capable of producing a rainbow. Moreover, he tested this conjecture in experiments with a magnified raindrop: a spherical flask filled with water. He was able to trace the path followed by the light rays that make up the rainbow.

In 1604, Johannes Kepler (1571–1630) followed up on the studies by Polish monk, mathematician, physicist, philosopher and theologian Witelo, but he was unable to establish the law of refraction.

The first theoretical descriptions of rainbows were those by René Descartes (who used ray tracing) and Baruch Spinoza (who used Cartesian analytical geometry), both in the 1620s. Around 1666, Isaac Newton improved upon Spinoza’s calculation, using calculus and analytical geometry, and verified the calculations with the most accurate measurements up to that time.
Theodoric of Freiberg on rainbow phenomenon