His explanation for his observations rested on his belief that each of the two knives repelled the rays as they passed by, thereby producing two sets of three exterior fringes".Diffraction is the line of study that helped in the development of precise spectrometers, hence helping Astronomy take great leaps. Thus Newton - in a beautifully detailed description - clearly reported observations on light bending into the geometric shadow. Closing the gap still further, Newton found that beginning at separations somewhat greater than 1/400 inch the fringes began to disappear, first the two outer fringes, then the two middle fringes, and finally the two inner fringes. At smaller knife separations, these fringes began to spread out. When they were quite close together Newton observed a bright line in the center of the pattern with three colored fringes on either side of it. Thus he placed two knives with their edges opposite each other and gradually decreased the space between them. " This keen observation (which was understood only after the work of Young and Fresnel) led Newton to further experiments which we would now classify as "single slit diffraction" experiments. Stuewer in Critical Analysis of Newton's Work on Diffraction writes: And he did observe " three Parallel Fringes orīands of Colour’d Light", for which he did not really have a good explanation. He did not seem to know about Grimaldi's second experiment, but one observation concerning the knife's shadow led him to a "single slit" variant anyway. He renamed diffraction into "inflexion" (Fabri did not use Grimaldi's term, its return is due to Fresnel), seriae lucidae into "fringes", and came up with a theory of light repulsion by matter to reconcile the effect with his corpuscular theory. Some time between then and the publication of Opticks (1704) Newton did perform detailed experiments extending those of Grimaldi (c. Hooke did not mention anything about colors. ![]() Newton read the book, but did not pay much attention to it even after Hooke presented to the Royal Society in 1675 his own observation of " a Penumbra or darker Ring encompassing the lighter Circle" obtained by passing sunlight through a small hole, and noted that " ‘it could not proceed from a Penumbra caused by the bigness of the Hole upon the common Principles, that is, from the Supposition of the Rays from every point of the Sun proceeding in strait Lines". Grimaldi's book was little known, but his experiments were popularized in Fabri's Dialogi Physici (1669), which found its way into Newton's hands in 1672, when Collins sent him a copy, see Hall's Beyond the Fringe: Diffraction as seen by Grimaldi, Fabri, Hooke and Newton. ![]() ![]() But in a second experiment he put the cone through a second small hole, and observed what is now called "single slit" diffraction, see Crosswell, Diffraction: the first recorded observation. In the more commonly cited experiment he placed a sharp knife into the cone of light passed into a dark room through a small hole, and observed knife's anomalously wide shadow along with the colored bands which he called seriae lucidae. The discovery of diffraction and the introduction of the term are due to Grimaldi, specifically his Physico-Mathesis de Lumine posthumously published in Bologna in 1665. Fresnel explained the near-field pattern before him, in 1815, but he was not the discoverer either. Fraunhofer is credited not with discovering the single slit diffraction, but with explaining the far-field diffraction pattern from a single slit.
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