Ruler and tube
Book I, Chapter 2, Paragraphs 6 to 9
In this experiment a long hollow tube is affixed to a ruler and the observer holds it up to one eye and looks at an object. As sections of the opening of the tube are blocked only those portions of the object that lie in a straight line between its surface of the object and the eye can be seen. This is used by Ibn al-Haytham to show that light travels in straight lines.
In the POV-Ray render a wooden block and hollow cylinder are used to form the ruler and tube. The oject being studied is a red cube. The camera is moved to where the eye would go and a cover for the tube can be moved into one of three positions: open, half-open, or closed.
Looking above the opening of the tube.
View down the fully open tube.
Tube is half closed: only half of the object can be seen.
With tube fully closed, the object cannot be seen at all.
al-Haytham uses this experiment to show that light travles in straight lines (as stated in paragraph 9):
“It follows from this experiment, with a necessity that dispels doubt, that sight does not perceive any visible object existing with it in the same atmosphere, this perception being not by reflection, except through the straight lines alone that can be imagined to extend between the surface of the object and the surface of the eye.”
Copyright © Tom Bickley and Tom Lancaster, Durham University.