All light—both primary and secondary—travels in rays, originating at a single point and moving in a straight line away from that point. The light of the sun, for example, travels from a point on the sun’s surface in a straight line through space. If a ray of sunlight strikes an opaque object on earth, that object will reflect it. The light reflected by an opaque object also forms a ray. It originates from a point on the surface of the object and travels away from it in a straight line.

Ibn al-Haytham points out that light radiates in all directions from its source. “The light shining from a self-luminous body into the transparent air,” he writes, “radiates from every part of the luminous body facing that air,…and it issues from every point on the luminous body in every straight line that can be imagined to extend in the air from that point.”

To prove that light radiates in a straight line from every point of a luminous object—not just the center, the ends, or the whole—Ibn al-Haytham describes an experiment similar to the one he used to prove that visual rays travel in straight lines. He starts with a sheet of copper with a large, circular hole in the center. Through this hole, he proposes that the experimenter slide “a well-straightened cylindrical tube of regular circularity and convenient length.” One end of the tube is open. The other end is closed, but punctured by an aperture, which should “not exceed the thickness of a needle.” The experimenter then holds a candle up to the open end of the cylinder “in the darkness of night” and holds an opaque object up to the aperture at the other end. Only a small amount of the light from the flame passes through the aperture. The rest of the light is blocked by the sheet of copper.

Then, he suggests, “the experimenter should…gently move the flame so another part of it may face the hole, and then inspect the body opposite.” As the flame moves, the light projected onto the opaque object changes. For example, when the tip of the flame is opposite the aperture, the light on the object appears weak. When the center of the flame is opposite the aperture, the light on the object appears bright. “Therefore,” he concludes, “it appears from this experiment that light radiates from each part of the fire.”

What is true of primary light is also true of secondary light. “From the light that shines on any body, light radiates in every opposite direction,” Ibn al-Haytham writes. This was an especially important discovery because it explains why vision remains steady even when the viewer’s eye moves. According to Ibn al-Haytham’s theory, a single point on any object radiates light rays into the air in all directions. If one of the rays enters the eye, it enables the viewer to see that point on the object. That same point also sends out countless rays of light that do not enter the eye. If the viewer moves his or her head slightly, the ray that originally entered the eye will miss it, but another ray, originating from the same point, will enter it. Since there are an infinite number of rays radiating from a single point, they will continue to steam into the eye as the viewer moves, providing the viewer with an uninterrupted view of that point.

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Copyright © 2008 by Bradley Steffens

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