Over the rainbow

Recently, I was lucky enough to visit the mighty Victoria Falls. As I stood at the falls’ edge drenched in spray, I spotted double rainbows formed by sunlight being refracted through the water droplets.

A rainbow, with a fainter secondary companion above, at Victoria Falls. (Alison Boyle)

One of the first people to explain how rainbows form was the Persian mathematician Kamal al-Din al-Farisi, who was born around 1260. Using a glass sphere filled with water to represent a raindrop, he showed that sunlight is bent as it enters the drop, reflects off the back of the drop, and is bent again on its way out. If rays are reflected twice inside the drop, a secondary rainbow is formed with the colours reversed. Here’s a more detailed explanation. Around the same time Theorodic of Freiberg performed a similar experiment. The two were not in contact, but both had been influenced by Ibn al-Haytham‘s Book of Optics. You can find out more about al-Farisi and al-Haytham in the 1001 Inventions exhibition.

Rainbows have fascinated people for centuries, as this illustration from 1535 shows. (Science Museum)

Isaac Newton explained that the rainbow’s colours arise as a result of white light being split into its constituent colours. Many people will have childhood memories of making a Newton colour wheel with a disc of cardboard and a pencil. Here’s a late 19th century version.

A 19th century demonstration apparatus. (Science Museum)

As our understanding of the nature of light has continued to change, so has our understanding of the rainbow. For a detailed account of how people have portrayed rainbows in science and beyond, check out Raymond Lee and Alastair Fraser’s The Rainbow Bridge: Rainbows in Art, Myth and Science.

2 thoughts on “Over the rainbow

  1. izzy

    Ok, I’m confused now…I always thought the first one to come up with a scientific explanation for the rainbow was Roger Bacon? He used similar methods as described above to come up with his theories (and they were considered heretical!), but I can’t remember if he came up his ideas were before or after the scientists/philosophers mentioned here, or if not, was influenced by them?

    Reply
  2. Alison Boyle, Curator of Astronomy and Modern Physics Post author

    Hi Izzy

    Bacon was the first to show that the maximum elevation of the rainbow is 42 degrees, in his Opus Majus of c. 1267, but it was not until a few decades later that al-Farisi and Theodoric outlined the refraction/reflection/refraction mechanism.

    Bacon had read Latin translations of al-Haytham’s work; I’m not sure if we have any evidence that Theodoric or al-Farisi read Bacon although it’s not unlikely that Theodoric might have had access to a manuscript copy of the Opus Majus through the Church.

    Regards
    Alison

    Reply

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