Ibn al-Haytham was not content to assert that reflected light radiates in all directions; he was determined to prove it. To do so, he devised an ingenious demonstration involving darkened rooms and the light of dawn. He starts with a rectangular building with outer walls that face north, south, east, and west. The eastern wall, he writes, should have “an opening or door at the top of the wall” that allows sunlight to enter the room, striking the western wall. Directly across from the western wall is the opening to a darkened chamber. “The experimenter should observe the place when morning light shines on that wall through the opening opposite, which should be fairly wide. He will find the chamber illuminated by that light, and the light in the chamber weaker than the light on that wall. Then, as the light on the wall grows stronger, so will the light in the chamber.” In other words, the darkened chamber is being lit indirectly, by light reflected by the sunlit wall.

Ibn al-Haytham then proposes a second chamber positioned inside the first chamber. He finds that it, too, is illuminated. The only light entering the building comes through the opening in the eastern wall. This light illuminates the western wall, which reflects light in all directions, illuminating not only the eastern wall, but also the floor, the ceiling, and all parts of the room, including the back wall of the darkened chamber. Every illuminated point in the room, in turn, sends out light rays in all directions, so that light enters not only the darkened chamber, but also a second chamber within it. The light grows weaker each time it is reflected, but it still reaches the inner chamber.

One of Ibn al-Haytham’s most important achievements was his decision to investigate the implications of his own findings. In doing so, he helped to develop what would later become known as the hypothico-deductive method of inquiry. This method states that a possible explanation, or hypothesis, cannot be considered to be proven true unless the consequences that follow from it are also proven to be true.

Ibn al-Haytham’s theory of vision suggested that light rays emanate in all directions from all illuminated objects. If this really is the case, Ibn al-Haytham deduced, each light ray must intersect, or cross, many other light rays. If intersecting light rays have any effect on each other, he reasoned, “it follows that these colours and lights will be mixed in the atmosphere and in the transparent bodies and will have reached the eye mixed; and they will affect the body of the eye while they are mixed, and thus neither the colours of the visible objects nor the objects [themselves] will be distinguished by the eye.” Everyday experience suggest that this does not happen. The eye can view several objects at the same time with complete clarity. Therefore, Ibn al-Haytham concluded, light rays must be able to intersect with each other without interference. Ibn al-Haytham understood that he had prove this consequence to be true, or else his entire theory would be in doubt. To prove that light rays intersect without affecting each other, he designed what would become the most famous experiment in The Book of Optics.

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