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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Interference. History and essence of scientific discovery
Directory / The most important scientific discoveries In ancient times, observing the behavior of light, they thought that two light beams, intersecting, continue to go their own way as if nothing had happened. Such observations strengthened the belief in the incorporeality, immateriality of light. But is this really what happens? Newton he was the first to set up an experiment on observing the interaction, or, as opticians say, the interference of light rays with each other. He created a wedge-shaped air gap by placing a thin lens (convex surface down) on a flat glass plate. Then the scientist illuminated the gap, first with white light, and then in turn with other main colored rays. Newton noted that the rays, reflected from the glass boundaries of the air wedge, obviously interacted with each other. When illuminated with white light, alternating color and iridescent rings appeared in the gap. When colored rays were passed through the gap, previously obtained with the help of a prism, light and dark rings appeared in it. Newton left these experiments without his customary detailed conclusions. Apparently, the scientist decided that there are hidden phenomena that require additional research that he could not carry out. Only in the XNUMXth century did two outstanding researchers, Jung and Fresnel, come to science and "complete" the building of classical optics laid down by Newton. Thomas Young (1773–1829), versatile scientist, physician by profession, a man of very versatile interests - a gymnast and musician, and also known as an Egyptologist. There is an interesting story related to him. At the age of fourteen, Thomas was asked to repeat a few phrases in English to see if he could write well. The young man stayed longer than usual in the testing room. The new teacher of Thomas Young was ready to laugh at the incompetence. However, when the student handed him a piece of paper, there the given phrases were not only rewritten, but also translated into nine (!) Different languages. In his first work on optics, Jung showed that the lens of the human eye is a lens with variable curvature. Special muscles stretch and compress the lens, allowing you to get a sharp image of both distant and close objects on the retina. Jung was only twenty years old when he performed this opto-medical examination. The Royal Society immediately elected him a member. To Jung's critical mind, Newton's theory seemed completely unsatisfactory. Especially unacceptable, he considered the constancy of the speed of light particles, regardless of whether they are emitted by such a tiny source as a smoldering ember, or such a huge source as the Sun. And most of all, the Newtonian theory of "attacks" seemed to him unclear and insufficient, with the help of which Newton tried to explain the coloring of thin plates. After reproducing this phenomenon and reflecting on it, Jung came up with a brilliant idea about the possibility of interpreting this phenomenon as a superposition of light reflected from the first surface of a thin plate and light transmitted into the plate, reflected from its second surface and then exited through the first. Such a superimposition could lead to a weakening or strengthening of the incident monochromatic light. It is not known exactly how Jung came up with his idea of superposition. It is likely that this happened as a result of the study of sound beats, in which there is a periodic increase and decrease in the sound perceived by the ear. Be that as it may, in four papers presented to the Royal Society from 1801 to 1803, combined a few years later in the summarizing work "Course of lectures on natural philosophy and mechanical art", published in London in 1807, Jung gives the results of his theoretical and experimental studies. He quotes several times from sentence XXIV of the third book of Newton's Principia, in which the anomalous tides observed by Halley in the Philippine archipelago are explained by Newton as the result of superposition of waves. From this particular example, Jung introduces the general principle of interference. “Imagine a series of identical waves running across the surface of a lake at a certain constant speed and entering a narrow channel leading to the outlet of the lake. Imagine further that, for some other similar reason, another series of waves of the same magnitude is excited, coming to that same channel at the same speed simultaneously with the first system of waves.Neither of these two systems will disturb the other, but their actions will add up: if they approach the channel in such a way that the vertices of one system of waves coincide with the vertices of the other system, then they together form a collection of waves of greater magnitude, but if the tops of one system of waves are located in the places of failures of another system, then they will exactly fill these failures and the surface of the water in the channel will remain even. light; and this superposition I call the general law of interference of light. To obtain interference, both light beams must come from the same source (so that they have exactly the same period), after passing a different path, they must fall into the same point, and also go almost parallel there. Hence, Jung continues, when two parts of light of common origin enter the eye along different paths, traveling in almost the same direction, the beam acquires maximum intensity, provided that the difference in the paths of the rays is equal to a multiple of some certain length, and has a minimum intensity in the intermediate case. This characteristic length is different for light of different colors. In 1802, Jung reinforced his principle of interference with the classical experiment "with two holes", possibly influenced by a similar experiment by Grimaldi, which, however, did not lead to the discovery of interference due to the peculiarities of the installation used. Young's experience is well known: in a transparent screen, two closely spaced holes are pierced with the tip of a pin, which are illuminated by sunlight passing through a small hole in the window. Two light cones formed behind an opaque screen, expanding due to diffraction, partially overlap, and in the overlapping part, instead of giving a uniform increase in illumination, form a series of alternating dark and light bands. If one hole is closed, then the fringes disappear and only diffraction rings from the other hole appear. These bands also disappear when both holes are illuminated (as in Grimaldi's experiment) directly by sunlight or by an artificial light source. Invoking the wave theory, Jung very simply explains this phenomenon. Dark bands are obtained there, says the scientist, where the dips of the waves that have passed through one hole are superimposed on the crests of the waves that have passed through another hole, so that their effects cancel each other out; light rims are obtained where two crests or two dips of waves that have passed through both holes add up. This experience allowed Jung to measure the wavelength for various colors: he obtained a wavelength of 0,7 microns for red light and 0,42 microns for extreme violet. These are the first measurements of the wavelength of light in the history of physics, and their amazing accuracy should be noted. From his principle of interference, Jung deduced a number of different consequences. He considered the phenomena of coloring thin layers. The scientist explained them down to the smallest detail. Jung derived the empirical laws found by Newton, and, considering the frequency of light of a given color to be constant, explained the compaction of the rings in Newton's experiment when replacing the air gap between lenses with water by a decrease in the speed of light in a more refractive medium. It is interesting to note that Jung owns the term "physical optics", which is used to refer to studies of "... light sources, the speed of its propagation, its interruption and attenuation, its splitting into different colors, the influence of different atmospheric densities on it, meteorological phenomena related to light, the special properties of certain substances in relation to light. Young's work, representing the most significant contribution to the theory of optical phenomena since the time of Newton, was perceived by the physicists of that time with distrust, and in England they were even subjected to rude ridicule. This was partly due to the fact that Jung tried to apply the principle of interference to clearly non-interference phenomena, and partly to some vagueness of presentation, which is still felt now and which must have been even more felt in those days, and partly, as Jung reproached later Laplace, the fact that Jung was sometimes satisfied with insufficiently rigorous, and sometimes superficial experiments. Augustin Fresnel (1788–1827), a road engineer, who began to be interested in science relatively late, also proceeded from the idea of light as a wave motion of the ether. The “good genius” of Fresnel, Academician Francois Arago, who noticed the scientist’s outstanding talent in time and helped him all his life, nevertheless wrote in his memoirs: “Augustin Fresnel studied so slowly that for eight years he could hardly read ... He never felt an inclination to learn languages, disliked knowledge based on mere memory, and memorized what was clearly and convincingly proven. At first, Fresnel worked in the rural wilderness. He had no idea about Jung's experiments, so he repeated them. And Fresnel gave an explanation of light bending around obstacles similar to Jung's. Later, while already working in Paris, Fresnel received mathematical equations that accurately describe the optical processes occurring at the boundary of two different optical media. Various Fresnel formulas are so often used in optical work that they undoubtedly occupy the first place in this indicator. Fresnel proposed to create an interference pattern by directing sunlight onto a screen using two mirrors set at a slight angle to each other. A well-known scientist, the author of many university textbooks on physics, Robert Pohl proposed for a large audience to create interference by directing light onto a thin mica plate. The light reflected by the plate hits a large screen, on which the interference fringes are clearly visible. The phenomenon of interference is widely used in devices called interferometers. Interferometers can serve a variety of purposes, for example, to control the cleanliness of metal surfaces. Author: Samin D.K.
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