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refraction and reflection
Alhazen’s theory of rainbows (clouds were huge concave reflecting mirrors) was copied from Roman philosopher Seneca the Younger’s first choice explanation.
[Seneca the Younger wrote that rainbows appear when water is sprayed, but Alhazen did not pay attention to that observation of Seneca the Younger.]
A book written by either the Shia Sevener scholar Avicenna (Ibn Sīnā) (980 – 1037) or by al-Qarafi copied the incorrect second choice explanation of Seneca the Younger that reflection but not refraction caused rainbows.
Sun Sikong (1015–1076) in China was the first to propose the nearly correct theory that the rainbow (he called it the “nine lights”) was caused by refraction of light by crystals of ice in the air.
In the 11/h century, Chinese scholars Sun Sikong and Shen Kuo were the first to propose the correct theory that rainbows were caused by rays of sunlight being refracted by raindrops.
Qutb al-Din al-Shirazi (1236–1311) independently proposed a similar explanation. Al-Shirazi’s student, Kamāl al-Dīn al-Fārisī (1267–1319), using a camera obscures and a glass sphere filled with water, discovered how the ray of light was refracted twice to form the rainbow.
Image by KES47, via Wikimedia Commons.
image credit https://en.wikipedia.org/wiki/File:Rainbow1.svg