Light in a straight line, and the rainbow

Look at the expectation: it names properties of light. Ask what the experiment changes and what it keeps the same, and which property the observer can read from the wall.

A model makes a hard-to-see thing easy to handle, and it also leaves things out. Ask what the sphere gives the experimenter and what it cannot show.

Read one experiment closely. In the sixth chapter of the first book of the Manazir, Ibn al-Haytham takes several candles placed at different points facing a single hole that opens into a dark place. On the wall opposite appear separate lights, as many as the candles, each on the straight line that runs from its candle through the hole. Cover one candle and only the light opposite it vanishes; uncover it and the light comes back ². He gives a practical way to do it, with a door of two leaves opened only slightly ³. Since all the lights meet at the hole and then part again after it, he concludes that lights do not mix in the air or in other transparent bodies ⁴. The record of the dark room adds that each version of the test was meant to be repeated by the reader.

The museum's records hold no passage in which Ibn al-Haytham says why he studied light, and they do not supply one for him. What they show is the frame he worked in. The first book of the Manazir opens with the basmala ⁵. The TDV encyclopaedia lists, besides his works in mathematics, astronomy and optics, treatises in which he answered Abu Bakr al-Razi's criticism of prophethood and took issue with the Mu'tazila on the divine attributes ⁶. The man who tested light with candles and a ruler, then, also wrote on theology. How he connected the two is a question for his own texts; the records read here do not answer it.

Was there one Ibn al-Haytham or two? The TDV encyclopaedia reports that Roshdi Rashed argued that the sources join two different men named Ibn al-Haytham, and that A. I. Sabra rejected this view in 1998 ⁶. The museum's record reports both positions and decides nothing.

Ibn al-Haytham is often called the inventor of the camera obscura. No source the museum has read names a first inventor of the dark room, and he does not claim the title: he presents light through holes as a familiar sight and turns it into a test ⁷. The museum therefore records him as an improver. What the TDV encyclopaedia credits him with is devising the dark-room experiment to prove that light travels in straight lines ⁶.

The experiment reaches us through manuscripts, not through a surviving instrument. The editor of the Arabic text lists the manuscripts he used; one of them, Ayasofya MS 2448 in Istanbul, contains all seven books and was copied in 869 AH, 1464–1465 CE ⁸. About twenty Latin manuscripts survive, at least seven of them from the thirteenth century ⁹. A second line of evidence is al-Farisi's revision of the book, which is in print ⁶. Ask of each witness how far it stands from the author's own hand.

The museum records Ibn al-Haytham as an improver of the dark room, and the TDV encyclopaedia calls his experiment the basis of today's photographic camera. Take a position and answer the strongest objection to it: does turning a familiar sight into a test that any reader can repeat deserve the word "invention"?

• Ibn al-Haytham's dark room: every version of the test, with the page of the Manazir it comes from.
• Ibn al-Haytham: his life and works, and the question of whether the sources describe one man or two.
• al-Farisi's water-sphere rainbow model: the dark room used again, with a glass sphere in place of a raindrop.
• Optics: the science these experiments belong to.