Ibn al-Haytham proposes that the experimenter position several lamps at various points in the same area, “all being opposite a single aperture leading to a dark place.” On the other side of the aperture is a blank wall where the experimenter can observe the effects of the light passing through the aperture. He finds that the various lights pass through the aperture along straight paths and appear separately on the blank wall. “If one of the lamps is screened,” Ibn al-Haytham observes, “only the light opposite that lamp in the dark will vanish. When the screen is moved away from the lamp, that light will return to its place. Whichever lamp is screened, only the light facing it in the [dark] place will disappear. When the screen is removed, the light will return to its place.”

Ibn al-Haytham then proposes a variation on the same experiment, this one employing more lamps and “a chamber with a two-panel door in a dark night,” which yields the same results as his original experiment. His results confirmed, Ibn al-Haytham reasons that “all the lights that appear in the dark place have reached it through the aperture alone…therefore the lights of all those lamps have come together at the aperture, then separated after passing through it. Thus, if lights blended in the atmosphere, the lights of the lamps meeting at the aperture would have mixed in the air at the aperture and in the air preceding it before they reached the aperture, and they would have come out so mingled together that they would not be subsequently distinguishable. We do not, however, find the matter to be so; rather the lights are found to come out separately, each being opposite the lamp from which it has arrived.

This experiment embodies all the elements of Ibn al-Haytham’s method of inquiry. He begins by stating the problem or question: do lights rays affect each other when they intersect? Next, he gathers information by observing how light behaves in various circumstances. Based on these observations, he offers a possible answer, or hypothesis: lights rays are able to intersect without being affected by each other. He then constructs a simple experiment to test this hypothesis, forcing the lights from different lamps to cross at a single point. After repeating his experiment and confirming his results, he finds that the evidence supports his hypothesis. This systematic, step-by-step approach, based on both sound logic and observed fact, would come to be known as the scientific method. It is the method of inquiry that scientists around the world continue to use in various incarnations, to this day.

Ibn al-Haytham’s lamp experiment also would gain fame because it offered the first full description of what would later become known as a camera obscura. Other scholars such as Aristotle, Theon of Alexandria, and al-Kindi had all described ways to project an image using an aperture. As a result, each has been credited with creating a kind of camera obscura. For example, Aristotle noted that sunlight traveling through small openings between the leaves of a tree, the holes of a sieve, the openings wickerwork, and even interlaced fingers will create circular patches of light on the ground. He also noted that during a solar eclipse, these patches of light—which are actually images of the sun—will change shape. He even built a box with a small hole to let in sunlight to better observe this phenomenon.

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Copyright © 2008 by Bradley Steffens

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