Chasing rainbows

Around 700 years ago Theodoric was a Dominican monk living in Freiberg, Germany. Like many people before and since he was fascinated by the beauty of rainbows. In the late 13th century no-one understood what rainbows were, which made them all the more mysterious. The only explanations were those associated with religion - the sign of God's promise to Noah - or with magic. Theodoric noticed the way rainbows always seemed to be associated with sun and rain, and set out to see if there was a link between the light, the water and the beautiful colours in the sky. Amazingly for someone living in those times, he set about his investigations in a scientific way.

Theodoric had a hypothesis that rainbows might be caused by the interaction of light and water. He set up a whole series of experiments using water in a round bottomed flask. He shone light through them and found that he could not only produce a 'rainbow' but that he could also produce a good explanation of how the rainbow came about. He suggested that light is refracted by droplets which are at different positions in space. Theodoric noticed that only one colour is seen when the drop is observed from any one direction which led him to the conclusion that the colours of the rainbow arrive at the eye from different raindrops in the cloud. He differed here from the famous Greek Aristotle who thought that rainbows came from only from an entire cloud, an idea which had been accepted for centuries.

In modern terms Theodoric said that different colours are refracted at different angles and that is what causes the separation of the different colours in a rainbow. He also recognised that the rainbow consisted of two bows. In 1304 Theodoric published his ideas which were remarkably accurate. Sadly the work of this obscure German monk was not widely read, and it was centuries before his level of understanding was reached again. It was Descartes in 1656 who rediscovered the diffraction of light in a raindrop.

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