A dark room and a small hole

The expectation asks you to link a property of light to how a device works. Say what the light does on its way through the hole and what you would see on the wall.

Read the first version of the experiment, the one with sunlight. When sunlight enters a dark room through a hole, and the air of the room is clouded with dust or smoke, the light can be seen stretching in a straight line from the hole to the spot where it falls on the floor or wall ². Set an opaque body anywhere on that line and the light falls on the body and vanishes from the spot beyond; remove the body and the spot returns. Lay a straight ruler along the line between the hole and the spot and the light keeps to the ruler's edge: its path is perfectly straight ³. Ibn al-Haytham says the same holds for the light of the moon ³. The record of the dark room gives the other versions.

The record preserves no sentence in which Ibn al-Haytham says why he made this experiment. It does report the editor's description of the whole book as a study of the properties of light in its three states, travelling straight, reflected and bent, built on experiment, which Ibn al-Haytham called i'tibar, together with mathematical methods ⁴. The first book opens with the basmala ⁵. TDV 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. How he joined these two kinds of writing is not answered in the record.

A reader may take Ibn al-Haytham to be the inventor of the dark room. The record says no one source names a first inventor of the dark room, and that he does not claim the title: he presents light through holes as a familiar sight and turns it into a test ². The museum therefore records him as an improver ². TDV credits him with devising the dark-room experiment to prove that light travels in straight lines ⁶.

The record shows three limits. First, the Manazir carries no date of writing; the third list of his works runs to the end of 429 AH (1037–1038 CE), holds 92 titles and names the Manazir among them ⁷, and the editor believes Ibn al-Haytham may have drawn it up himself ⁸. Second, the Latin translation, through which the book reached Europe, lacks the first three chapters of the first book ⁹, so the dust and ruler passage of chapter 3 did not reach Latin readers ⁹. Third, the Arabic text is edited for the first three books only, by A. I. Sabra, Kuwait, 1983 ¹⁰.

The test lays a ruler along the light and sets an opaque body on its line ³. Which step would convince you more that light travels in a straight line, the cutting body or the ruler, why, and what would a doubter say in answer?

• Ibn al-Haytham's dark room: the sunlight, starlight, eclipse and candle versions of the test.
• Ibn al-Haytham: his life, his writings and the question of whether the sources describe one man or two.
• The Book of Optics: the book in which the experiments are set out.
• al-Farisi's water-sphere rainbow model: the dark room used again with a glass sphere.