DIFFRACTION, or INFLECTION, of the rays of light. It was observed by Grimaldi that if a beam of the sun's light be let into a dark room through a very sma)1 hole, the shadows of things in this light will be larger than they ought to be if the rays passed by the bodies in straight lines, and that these shadows have three parallel fringes, bands, or ranks of colored light adjacent to them.
This phenomenon was originally known under the name of diffraction, and was supposed to arise from the refraction of the atmosphere. This explanation was dis proved by the observations of Newton, who, from the conception which he was led to form of it, called the phenomenon the "inflection of the rays of light." It is now iden tified with a larger class of phenomena, which have been much more completely explained in the later development of the theory of light, and are assigned, on the hypotheses of Fresnel, to the interference of undulations. Sec INTERFERENCE. The observations and experiments of Newton on the subject. as detailed in the third book of his work on Optics. are, however, extremely interesting and instructive, and with regard to accurate observation and description, apart from the imperfect state of the theory, leave nothing to be desired.
Having made in a piece of lead a small hole with a pin, whose breadth was the 42d part of an inch, Newton let into the darkened chamber a beam of the sun's light. In this light, the shadows of all bodies were bordered with three parallel fringes or bands of colored light. The shadow of a hair, too, was found to be much broader than the hair itself, and fringes of light were observed within it.
Again admitting light into the darkened apartment by a hole a quarter of an inch wide, he allowed it to ,pass between two knife-edges parallel to one another. In this case, owing to the breadth of the hole by which the light was admitted, the fringes did not appear within the shadows of the knives until the knife-edges were brought to approach one another, when they appeared. By making the hole smaller through which the light was admitted, they became more distinct. "As the knife-edges con tinually approached one another, the fringes grew distiucter and larger, until they van ished. The outmost fringe vanished first, and the middlemost next, and the innermost last; and after they were all vanished, and the line of light which was in the middle between them was grown very broad, a shadow began to appear in the middle of this line, and divide it along the middle into two lines of light, and increased until the whole light vanished. This enlargement of the fringes was so great, that the rays
which go to the innermost fringe seemed to be bent above twenty times more when this fringe was ready to vanish titan when one of the knives was taken away." The order of these phenomena is then made clear by the following experiment: "I caused the edges of two knives to be ground truly straight, and pricking their points into a board, so that their edges might look towards one another, and meeting near their points, contain a rectilineal angle, I fastened their handles together with pitch, to make this angle invariable. The distance of the edges of the knives from one another at the distance of four inches from the angular point, where the edges of the knives met, was the eighth of an inch; and therefore the angle contained by the edges was about 1' 54'. The knives thus fixed together I placed in a beam of the sun's light, let into my darkened chamber through a hole the 42d part of an inch \vide, at the distance of 10 or 15 ft. from the hole." When the fringes of the shadows of the knives fell perpendicularly upon a paper at a great distance from the knives, they were in the form of hyperbolas.
The best mode for exhibiting the phenomena of diffraction, and that now generally adopted for that purpose, is as follows: The rays sunlight being reflected horizontally through an aperture into a darkened apartment, re re concentrated by a combination of lenses to a very small focus. By this means the ight is made to diverge from a very small circle, with the advantage of a greater concentration of light than is obtained by simply admitting the sunlight through a small aperture. The edges of the shadows of every object placed within the cone of light diverging from this focus, will exhibit the fringes above described.
By means of metal-leaf arranged upon a plate of glass, shadows can now be thrown upon a screen, so as to exhibit at once all the most peculiar phenomena of this class.
On the assumption of Fresnel, the explanation of the whole phenomena above described is most complete and satisfactory, the fringes and dark lines being produced by the undulations alternately strengthening or destroying each other. A very beautiful experiment, devised by Fresnel for the purpose, is found to furnish a complete verifica tion of the theory. See INTERFERENCE.